diff --git a/include/cantera/thermo/PureFluidPhase.h b/include/cantera/thermo/PureFluidPhase.h
index 1cfe75204..62a99b6e4 100644
--- a/include/cantera/thermo/PureFluidPhase.h
+++ b/include/cantera/thermo/PureFluidPhase.h
@@ -553,15 +553,6 @@ protected:
     //! Sets the state using a TPX::TV call
     void setTPXState() const;
 
-    //! Carry out a internal check on tpx, it may have thrown an error.
-    /*!
-     * @param v Defaults to zero
-     */
-    void check(doublereal v = 0.0) const;
-
-    //! Report errors in the TPX level
-    void reportTPXError() const;
-
 private:
 
     //! Pointer to the underlying tpx object Substance that does the work
diff --git a/include/cantera/tpx/Sub.h b/include/cantera/tpx/Sub.h
index 630ae2d28..ef36229cc 100644
--- a/include/cantera/tpx/Sub.h
+++ b/include/cantera/tpx/Sub.h
@@ -19,22 +19,18 @@
 #ifndef TPX_SUB_H
 #define TPX_SUB_H
 
+#include "cantera/base/ctexceptions.h"
+
 #include <iostream>
 #include <string>
 
 namespace tpx
 {
 
-class TPX_Error
+class TPX_Error : public Cantera::CanteraError
 {
 public:
-    TPX_Error(std::string p, std::string e) {
-        ErrorMessage = e;
-        ErrorProcedure = p;
-    }
-    virtual ~TPX_Error() {}
-    static std::string ErrorMessage;
-    static std::string ErrorProcedure;
+    TPX_Error(std::string p, std::string e) : CanteraError(p, e) { }
 };
 
 
@@ -50,14 +46,6 @@ const int VT = -12, PH = -34, PS = -54, VP = -42, PT = -14, VU = -62,
           TS = -51, VS = -52, PU = -64, HV = -23, HT = -13, HS = -53,
           XP = -47, XT = -17;
 
-const int NoConverge = -900;
-const int GenError = -901;
-const int InvalidInput = -902;
-const int TempError = -800;
-const int PresError = -801;
-const int CKError = -802;
-
-
 const int Pgiven = 0, Tgiven = 1;
 
 const int EvalH = 3;
@@ -82,8 +70,6 @@ const int EvalRgas = 19;
 
 const double Undef = 999.1234;
 
-std::string errorMsg(int flag);
-
 class Substance
 {
 public:
@@ -205,17 +191,11 @@ public:
     void Set(int XY, double x0, double y0);
     void Set_meta(double phase, double pp);
 
-    int Error() {
-        return Err;
-    }
-
-
 protected:
 
     double T, Rho;
     double Tslast, Rhf, Rhv;
     double Pst;
-    int Err;
     double m_energy_offset;
     double m_entropy_offset;
     std::string m_name;
@@ -237,17 +217,6 @@ protected:
     int Lever(int itp, double sat, double val, int ifunc);
     void update_sat();
 
-    void set_Err(int ErrFlag) {
-        if (!Err) {
-            Err = ErrFlag;
-            //throw TPX_Error(""errorMsg(Err));
-        }
-    }
-    void clear_Err() {
-        Err = 0;
-    }
-
-
 private:
     void set_Rho(double r0);
     void set_T(double t0);
@@ -265,9 +234,6 @@ private:
     double v_here, P_here;
 };
 
-void Error(char* message, int flag, double val=Undef);
-void Mess(char* message);
-
 }
 
 
diff --git a/src/thermo/PureFluidPhase.cpp b/src/thermo/PureFluidPhase.cpp
index c6e2c2d3a..3be6435b9 100644
--- a/src/thermo/PureFluidPhase.cpp
+++ b/src/thermo/PureFluidPhase.cpp
@@ -134,63 +134,49 @@ doublereal PureFluidPhase::
 enthalpy_mole() const
 {
     setTPXState();
-    doublereal h = m_sub->h() * m_mw;
-    check(h);
-    return h;
+    return m_sub->h() * m_mw;
 }
 
 doublereal PureFluidPhase::
 intEnergy_mole() const
 {
     setTPXState();
-    doublereal u = m_sub->u() * m_mw;
-    check(u);
-    return u;
+    return m_sub->u() * m_mw;
 }
 
 doublereal PureFluidPhase::
 entropy_mole() const
 {
     setTPXState();
-    doublereal s = m_sub->s() * m_mw;
-    check(s);
-    return s;
+    return m_sub->s() * m_mw;
 }
 
 doublereal PureFluidPhase::
 gibbs_mole() const
 {
     setTPXState();
-    doublereal g = m_sub->g() * m_mw;
-    check(g);
-    return g;
+    return m_sub->g() * m_mw;
 }
 
 doublereal PureFluidPhase::
 cp_mole() const
 {
     setTPXState();
-    doublereal cp = m_sub->cp() * m_mw;
-    check(cp);
-    return cp;
+    return m_sub->cp() * m_mw;
 }
 
 doublereal PureFluidPhase::
 cv_mole() const
 {
     setTPXState();
-    doublereal cv = m_sub->cv() * m_mw;
-    check(cv);
-    return cv;
+    return m_sub->cv() * m_mw;
 }
 
 doublereal PureFluidPhase::
 pressure() const
 {
     setTPXState();
-    doublereal p = m_sub->P();
-    check(p);
-    return p;
+    return m_sub->P();
 }
 //====================================================================================================================
 void PureFluidPhase::
@@ -198,16 +184,11 @@ setPressure(doublereal p)
 {
     Set(tpx::TP, temperature(), p);
     setDensity(1.0/m_sub->v());
-    check();
 }
 //====================================================================================================================
 void PureFluidPhase::Set(int n, double x, double y) const
 {
-    try {
-        m_sub->Set(n, x, y);
-    } catch (tpx::TPX_Error) {
-        reportTPXError();
-    }
+    m_sub->Set(n, x, y);
 }
 //====================================================================================================================
 void PureFluidPhase::setTPXState() const
@@ -215,21 +196,6 @@ void PureFluidPhase::setTPXState() const
     Set(tpx::TV, temperature(), 1.0/density());
 }
 //====================================================================================================================
-void PureFluidPhase::check(doublereal v) const
-{
-    if (m_sub->Error() || v == tpx::Undef) {
-        throw CanteraError("PureFluidPhase",string(tpx::errorMsg(
-                               m_sub->Error())));
-    }
-}
-//====================================================================================================================
-void PureFluidPhase::reportTPXError() const
-{
-    string msg = tpx::TPX_Error::ErrorMessage;
-    string proc = "tpx::"+tpx::TPX_Error::ErrorProcedure;
-    throw CanteraError(proc,msg);
-}
-//====================================================================================================================
 
 doublereal PureFluidPhase::isothermalCompressibility() const
 {
@@ -468,13 +434,8 @@ doublereal PureFluidPhase::critDensity() const
 /// saturation temperature
 doublereal PureFluidPhase::satTemperature(doublereal p) const
 {
-    try {
-        doublereal ts = m_sub->Tsat(p);
-        return ts;
-    } catch (tpx::TPX_Error) {
-        reportTPXError();
-        return -1.0;
-    }
+    doublereal ts = m_sub->Tsat(p);
+    return ts;
 }
 //====================================================================================================================
 void PureFluidPhase::setState_HP(doublereal h, doublereal p,
@@ -482,7 +443,6 @@ void PureFluidPhase::setState_HP(doublereal h, doublereal p,
 {
     Set(tpx::HP, h, p);
     setState_TR(m_sub->Temp(), 1.0/m_sub->v());
-    check();
 }
 //====================================================================================================================
 void PureFluidPhase::setState_UV(doublereal u, doublereal v,
@@ -490,7 +450,6 @@ void PureFluidPhase::setState_UV(doublereal u, doublereal v,
 {
     Set(tpx::UV, u, v);
     setState_TR(m_sub->Temp(), 1.0/m_sub->v());
-    check();
 }
 //====================================================================================================================
 void PureFluidPhase::setState_SV(doublereal s, doublereal v,
@@ -498,7 +457,6 @@ void PureFluidPhase::setState_SV(doublereal s, doublereal v,
 {
     Set(tpx::SV, s, v);
     setState_TR(m_sub->Temp(), 1.0/m_sub->v());
-    check();
 }
 //====================================================================================================================
 void PureFluidPhase::setState_SP(doublereal s, doublereal p,
@@ -506,29 +464,21 @@ void PureFluidPhase::setState_SP(doublereal s, doublereal p,
 {
     Set(tpx::SP, s, p);
     setState_TR(m_sub->Temp(), 1.0/m_sub->v());
-    check();
 }
 //====================================================================================================================
 // saturation pressure
 doublereal PureFluidPhase::satPressure(doublereal t) const
 {
     doublereal vsv = m_sub->v();
-    try {
-        Set(tpx::TV,t,vsv);
-        doublereal ps = m_sub->Ps();
-        return ps;
-    } catch (tpx::TPX_Error) {
-        reportTPXError();
-        return -1.0;
-    }
+    Set(tpx::TV,t,vsv);
+    doublereal ps = m_sub->Ps();
+    return ps;
 }
 //====================================================================================================================
 doublereal PureFluidPhase::vaporFraction() const
 {
     setTPXState();
-    doublereal x = m_sub->x();
-    check(x);
-    return x;
+    return m_sub->x();
 }
 //====================================================================================================================
 void PureFluidPhase::setState_Tsat(doublereal t, doublereal x)
@@ -537,7 +487,6 @@ void PureFluidPhase::setState_Tsat(doublereal t, doublereal x)
     setTPXState();
     Set(tpx::TX, t, x);
     setDensity(1.0/m_sub->v());
-    check();
 }
 //====================================================================================================================
 void PureFluidPhase::setState_Psat(doublereal p, doublereal x)
@@ -546,7 +495,6 @@ void PureFluidPhase::setState_Psat(doublereal p, doublereal x)
     Set(tpx::PX, p, x);
     setTemperature(m_sub->Temp());
     setDensity(1.0/m_sub->v());
-    check();
 }
 
 //====================================================================================================================
diff --git a/src/tpx/CarbonDioxide.cpp b/src/tpx/CarbonDioxide.cpp
index 2ceecb6e8..2d5c3e173 100644
--- a/src/tpx/CarbonDioxide.cpp
+++ b/src/tpx/CarbonDioxide.cpp
@@ -10,6 +10,9 @@
 #include "CarbonDioxide.h"
 #include <math.h>
 #include <string.h>
+#include "cantera/base/stringUtils.h"
+
+using namespace Cantera;
 
 namespace tpx
 {
@@ -320,8 +323,8 @@ double CarbonDioxide::Psat()
 
     double log, sum=0,P;
     if ((T < Tmn) || (T > Tc)) {
-        std::cout << " error in Psat " << TempError << std::endl;
-        set_Err(TempError); // Error("CarbonDioxide::Psat",TempError,T);
+        throw TPX_Error("CarbonDixoide::Psat",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     for (int i=1; i<=8; i++) {
         sum += F[i-1] * pow((T/Tp -1),double(i-1));
@@ -343,8 +346,8 @@ double CarbonDioxide::ldens()
 {
     double xx=1-(T/Tc), sum=0;
     if ((T < Tmn) || (T > Tc)) {
-        std::cout << " error in ldens " << TempError << std::endl;
-        set_Err(TempError);
+        throw TPX_Error("CarbonDixoide::ldens",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     for (int i=1; i<=6; i++) {
         sum+=D[i-1]*pow(xx,double(i-1)/3.0);
diff --git a/src/tpx/HFC134a.cpp b/src/tpx/HFC134a.cpp
index bf9f542e4..38db2834a 100644
--- a/src/tpx/HFC134a.cpp
+++ b/src/tpx/HFC134a.cpp
@@ -2,6 +2,9 @@
 
 #include "HFC134a.h"
 #include <math.h>
+#include "cantera/base/stringUtils.h"
+
+using namespace Cantera;
 
 namespace tpx
 {
@@ -153,7 +156,8 @@ double HFC134a::Pp()
 double HFC134a::Psat()
 {
     if ((T < Tmn) || (T > Tc)) {
-        set_Err(TempError);
+        throw TPX_Error("HFC134a::Psat",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     double x1 = T/Tc;
     double x2 = 1.0 - x1;
@@ -178,7 +182,8 @@ double HFC134a::Psat()
 double HFC134a::ldens()
 {
     if ((T < Tmn) || (T > Tc)) {
-        set_Err(TempError);
+        throw TPX_Error("HFC134a::ldens",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     double x1 = T/Tc;
     double x2 = 1.0 - x1;
diff --git a/src/tpx/Heptane.cpp b/src/tpx/Heptane.cpp
index e977725e3..573287d70 100644
--- a/src/tpx/Heptane.cpp
+++ b/src/tpx/Heptane.cpp
@@ -8,9 +8,12 @@
  */
 
 #include "Heptane.h"
+#include "cantera/base/stringUtils.h"
 #include <math.h>
 #include <string.h>
 
+using namespace Cantera;
+
 namespace tpx
 {
 
@@ -277,8 +280,8 @@ double Heptane::Psat()
 {
     double log, sum=0,P;
     if ((T < Tmn) || (T > Tc)) {
-
-        set_Err(TempError); // Error("Heptane::Psat",TempError,T);
+        throw TPX_Error("Heptane::Psat",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     for (int i=1; i<=8; i++) {
         sum += F[i-1] * pow((T/Tp -1),double(i-1));
@@ -299,7 +302,8 @@ double Heptane::ldens()
 {
     double xx=1-(T/Tc), sum=0;
     if ((T < Tmn) || (T > Tc)) {
-        set_Err(TempError);
+        throw TPX_Error("Heptane::ldens",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     for (int i=1; i<=6; i++) {
         sum+=D[i-1]*pow(xx,double(i-1)/3.0);
diff --git a/src/tpx/Hydrogen.cpp b/src/tpx/Hydrogen.cpp
index 849918cae..56f484d8f 100644
--- a/src/tpx/Hydrogen.cpp
+++ b/src/tpx/Hydrogen.cpp
@@ -2,6 +2,9 @@
 
 #include "Hydrogen.h"
 #include <math.h>
+#include "cantera/base/stringUtils.h"
+
+using namespace Cantera;
 
 namespace tpx
 {
@@ -221,7 +224,8 @@ double hydrogen::Pp()
 double hydrogen::ldens()
 {
     if ((T < Tmn) || (T > Tc)) {
-        set_Err(TempError);
+        throw TPX_Error("hydrogen::ldens",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     double x=1-T/Tc;
     double sum, term;
@@ -240,7 +244,8 @@ double hydrogen::Psat()
     double x = (1.0 - Tt/T)/(1.0 - Tt/Tc);
     double result;
     if ((T < Tmn) || (T > Tc)) {
-        set_Err(TempError);
+        throw TPX_Error("hydrogen::Psat",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     result = Fhydro[0]*x + Fhydro[1]*x*x + Fhydro[2]*x*x*x +
              Fhydro[3]*x*pow(1-x, alpha);
diff --git a/src/tpx/Methane.cpp b/src/tpx/Methane.cpp
index 3ffd51f08..423339612 100644
--- a/src/tpx/Methane.cpp
+++ b/src/tpx/Methane.cpp
@@ -1,9 +1,12 @@
 // Methane
 
 #include "Methane.h"
+#include "cantera/base/stringUtils.h"
 #include <math.h>
 #include <iostream>
+
 using namespace std;
+using namespace Cantera;
 
 namespace tpx
 {
@@ -193,7 +196,8 @@ double methane::Psat()
     double x = (1.0 - Tt/T)/(1.0 - Tt/Tc);
     double result;
     if ((T < Tmn) || (T > Tc)) {
-        set_Err(TempError);
+        throw TPX_Error("methane::Psat",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     result = Fmeth[0]*x + Fmeth[1]*x*x + Fmeth[2]*x*x*x +
              Fmeth[3]*x*pow(1-x, alpha);
@@ -208,7 +212,8 @@ double methane::ldens()
     double sum;
     double w;
     if ((T < Tmn) || (T > Tc)) {
-        set_Err(TempError);
+        throw TPX_Error("methane::ldens",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     w = (Tc - T)/(Tc - Tt);
     sum = Dmeth[0]*(1.0 - pow(w, 2.0/3.0)) + Dmeth[1]*(1.0 - pow(w, 4.0/3.0))
diff --git a/src/tpx/Nitrogen.cpp b/src/tpx/Nitrogen.cpp
index 23130d71d..931e34c90 100644
--- a/src/tpx/Nitrogen.cpp
+++ b/src/tpx/Nitrogen.cpp
@@ -1,9 +1,11 @@
 // Nitrogen
 
 #include "Nitrogen.h"
+#include "cantera/base/stringUtils.h"
 #include <math.h>
 #include <iostream>
 using namespace std;
+using namespace Cantera;
 
 namespace tpx
 {
@@ -209,7 +211,8 @@ double nitrogen::Psat()
     double lnp;
     int i;
     if ((T < Tmn) || (T > Tc)) {
-        set_Err(TempError);
+        throw TPX_Error("nitrogen::Psat",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     for (i=0, lnp=0; i<=7; i++) {
         if (i==3) {
@@ -227,7 +230,8 @@ double nitrogen::ldens()
 {
     double xx=1-T/Tc, sum=0;
     if ((T < Tmn) || (T > Tc)) {
-        set_Err(TempError);
+        throw TPX_Error("nitrogen::ldens",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     for (int i=0; i<=5; i++) {
         sum+=Dnn[i]*pow(xx,double(i)/3.0);
diff --git a/src/tpx/Oxygen.cpp b/src/tpx/Oxygen.cpp
index af6970557..d9433ae8e 100644
--- a/src/tpx/Oxygen.cpp
+++ b/src/tpx/Oxygen.cpp
@@ -1,8 +1,11 @@
 // Oxygen
 
 #include "Oxygen.h"
+#include "cantera/base/stringUtils.h"
 #include <math.h>
 
+using namespace Cantera;
+
 namespace tpx
 {
 
@@ -198,7 +201,8 @@ double oxygen::Psat()
     double lnp;
     int i;
     if ((T < Tmn) || (T > Tc)) {
-        set_Err(TempError);
+        throw TPX_Error("oxygen::Psat",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     for (i=0, lnp=0; i<=7; i++) {
         if (i==3) {
@@ -216,7 +220,8 @@ double oxygen::ldens()
 {
     double xx=1-T/Tc, sum=0;
     if ((T < Tmn) || (T > Tc)) {
-        set_Err(TempError);
+        throw TPX_Error("oxygen::ldens",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     for (int i=0; i<=5; i++) {
         sum+=Doxy[i]*pow(xx,double(i)/3.0);
diff --git a/src/tpx/Sub.cpp b/src/tpx/Sub.cpp
index 4772d7625..db1c977bd 100644
--- a/src/tpx/Sub.cpp
+++ b/src/tpx/Sub.cpp
@@ -3,51 +3,16 @@
  * D. Goodwin, Caltech Nov. 1996
  */
 #include "cantera/tpx/Sub.h"
+#include "cantera/base/stringUtils.h"
 #include <math.h>
 #include <fstream>
 #include <stdio.h>
 
 using std::string;
+using namespace Cantera;
 
 namespace tpx
 {
-
-static string fp2str(double x, string fmt="%g")
-{
-    char buf[30];
-    sprintf(buf, fmt.c_str(), x);
-    return string(buf);
-}
-
-/*
-    static string int2str(int n, string fmt="%d") {
-        char buf[30];
-        sprintf(buf, fmt.c_str(), n);
-        return string(buf);
-    }
-*/
-
-string TPX_Error::ErrorMessage = "";
-string TPX_Error::ErrorProcedure = "";
-
-string errorMsg(int flag)
-{
-    switch (flag) {
-    case NoConverge:
-        return "no convergence";
-    case GenError:
-        return "general error";
-    case InvalidInput:
-        return "invalid input";
-    case TempError:
-        return "temperature error";
-    case PresError:
-        return "pressure error";
-    default:
-        return "(unknown error)";
-    }
-}
-
 //-------------- Public Member Functions --------------
 
 Substance::Substance() :
@@ -57,7 +22,6 @@ Substance::Substance() :
     Rhf(Undef),
     Rhv(Undef),
     Pst(Undef),
-    Err(0),
     m_energy_offset(0.0),
     m_entropy_offset(0.0),
     kbr(0)
@@ -69,8 +33,7 @@ Substance::Substance() :
 /// computed directly from the underlying eos.
 double Substance::P()
 {
-    double ppp = (TwoPhase() ? Ps() : Pp());
-    return (Err ? Undef : ppp);
+    return TwoPhase() ? Ps() : Pp();
 }
 
 const double DeltaT = 0.000001;
@@ -85,7 +48,7 @@ double Substance::dPsdT()
     set_T(T + DeltaT);
     dpdt = (Ps() - ps1)/DeltaT;
     set_T(tsave);
-    return (Err ? Undef : dpdt);
+    return dpdt;
 }
 
 /// true if a liquid/vapor mixture, false otherwise
@@ -103,21 +66,20 @@ int Substance::TwoPhase()
 /// returned if v > Vcrit.
 double Substance::x()
 {
-    double xx, vv, vl;
+    double vv, vl;
     if (T >= Tcrit()) {
         return (1.0/Rho < Vcrit() ? 0.0 : 1.0);
     } else {
         update_sat();
         if (Rho <= Rhv) {
-            xx = 1.0;
+            return 1.0;
         } else if (Rho >= Rhf) {
-            xx = 0.0;
+            return 0.0;
         } else {
             vv = 1.0/Rhv;
             vl = 1.0/Rhf;
-            xx = (1.0/Rho - vl)/(vv - vl);
+            return (1.0/Rho - vl)/(vv - vl);
         }
-        return (Err ? Undef : xx);
     }
 }
 
@@ -126,10 +88,8 @@ double Substance::Tsat(double p)
 {
     double Tsave, p_here, dp, dt, dpdt, dta,
            dtm, tsat;
-    if (Err || (p <= 0.0) || (p > Pcrit())) {
-        throw TPX_Error("Substance::Tsat","illegal pressure value");
-        set_Err(PresError);
-        return Undef;
+    if (p <= 0.0 || p > Pcrit()) {
+        throw TPX_Error("Substance::Tsat", "illegal pressure value");
     }
     int LoopCount = 0;
     double tol = 1.e-6*p;
@@ -141,9 +101,6 @@ double Substance::Tsat(double p)
         T = 0.5*(Tcrit() - Tmin());
     }
     do {
-        if (Err) {
-            break;
-        }
         if (T > Tcrit()) {
             T = Tcrit() - 0.001;
         }
@@ -163,13 +120,12 @@ double Substance::Tsat(double p)
         LoopCount++;
         if (LoopCount > 100) {
             T = Tsave;
-            set_Err(NoConverge);
-            return Undef;
+            throw TPX_Error("Substance::Tsat", "No convergence");
         }
     } while (fabs(dp) > tol);
     tsat = T;
     T = Tsave;
-    return (Err ? Undef : tsat);
+    return tsat;
 }
 
 
@@ -187,8 +143,6 @@ void Substance::Set(int XY, double x0, double y0)
 {
     double temp;
 
-    clear_Err();  // clear error flag
-
     /* if inverted (PT) switch order and change sign of XY (TP = -PT) */
     if (XY < 0) {
         double tmp = x0;
@@ -274,34 +228,35 @@ void Substance::Set(int XY, double x0, double y0)
 
     case PX:
         temp = Tsat(x0);
-        if ((y0 >= 0.0) && (y0 <= 1.0) && (temp < Tcrit())) {
+        if (y0 > 1.0 || y0 < 0.0) {
+            throw TPX_Error("Substance::Set",
+                            "Invalid vapor fraction, " + fp2str(y0));
+        } else if (temp >= Tcrit()) {
+            throw TPX_Error("Substance::Set",
+                            "Can't set vapor fraction above the critical point");
+        } else {
             set_T(temp);
             update_sat();
             Rho = 1.0/((1.0 - y0)/Rhf + y0/Rhv);
-        } else {
-            set_Err(InvalidInput);
         }
         break;
 
     case TX:
-        if ((y0 >= 0.0) && (y0 <= 1.0) && (x0 < Tcrit())) {
+        if (y0 > 1.0 || y0 < 0.0) {
+            throw TPX_Error("Substance::Set",
+                            "Invalid vapor fraction, " + fp2str(y0));
+        } else if (x0 >= Tcrit()) {
+            throw TPX_Error("Substance::Set",
+                            "Can't set vapor fraction above the critical point");
+        } else {
             set_T(x0);
             update_sat();
             Rho = 1.0/((1.0 - y0)/Rhf + y0/Rhv);
-        } else {
-            set_Err(InvalidInput);
         }
         break;
 
     default:
-        set_Err(InvalidInput);
-    }
-    if (Err) {
-        T = Undef;
-        Rho = Undef;
-        Tslast = Undef;
-        Rhf = Undef;
-        Rhv = Undef;
+        throw TPX_Error("Substance::Set", "Invalid input.");
     }
 }
 
@@ -312,7 +267,7 @@ void Substance::set_Rho(double r0)
     if (r0 > 0.0) {
         Rho = r0;
     } else {
-        set_Err(InvalidInput);
+        throw TPX_Error("Substance::set_Rho", "Invalid density: " + fp2str(r0));
     }
 }
 
@@ -321,9 +276,7 @@ void Substance::set_T(double t0)
     if ((t0 >= Tmin()) && (t0 <= Tmax())) {
         T = t0;
     } else {
-        throw TPX_Error("Substance::set_T",
-                        "illegal temperature value "+fp2str(t0));
-        set_Err(TempError);
+        throw TPX_Error("Substance::set_T", "illegal temperature: " + fp2str(t0));
     }
 }
 
@@ -334,7 +287,6 @@ void Substance::set_v(double v0)
     } else {
         throw TPX_Error("Substance::set_v",
                         "negative specific volume: "+fp2str(v0));
-        set_Err(InvalidInput);
     }
 }
 
@@ -343,8 +295,6 @@ double Substance::Ps()
     if (T < Tmin() || T > Tcrit()) {
         throw TPX_Error("Substance::Ps",
                         "illegal temperature value "+fp2str(T));
-        set_Err(TempError);
-        return Undef;
     }
     update_sat();
     return Pst;
@@ -427,12 +377,7 @@ void Substance::update_sat()
         }
 
         if (i >= 20) {
-            Pst = Undef;
-            Rhv = Undef;
-            Rhf = Undef;
-            Tslast = Undef;
             throw TPX_Error("substance::update_sat","no convergence");
-            set_Err(NoConverge);
         } else {
             Pst = pp;
             Tslast = T;
@@ -455,7 +400,7 @@ double Substance::vprop(int ijob)
     case EvalP:
         return Pp();
     default:
-        return Undef;
+        throw TPX_Error("Substance::vprop", "invalid job index");
     }
 }
 
@@ -479,25 +424,22 @@ int Substance::Lever(int itp, double sat, double val, int ifunc)
             return 0;
         }
         psat = sat;
-        T = Tsat(psat);
-        if (T == Undef) {
-            Err = 0;
+        try {
+            T = Tsat(psat);
+        } catch (TPX_Error&) {
+            // Failure to converge here is not an error
             T = Tsave;
             Rho = Rhosave;
             return 0;
         }
     } else {
         throw TPX_Error("Substance::Lever","general error");
-        set_Err(GenError);
-        return GenError;
     }
     Set(TX, T, Vapor);
     Valg = vprop(ifunc);
     Set(TX, T, Liquid);
     Valf = vprop(ifunc);
-    if (Err) {
-        return Err;
-    } else if ((val >= Valf) && (val <= Valg)) {
+    if (val >= Valf && val <= Valg) {
         xx = (val - Valf)/(Valg - Valf);
         vv = (1.0 - xx)/Rhf + xx/Rhv;
         set_v(vv);
@@ -525,10 +467,6 @@ void Substance::set_xy(int ifx, int ify, double X, double Y,
     double v_save = 1.0/Rho;
     double t_save = T;
 
-    if (Err) {
-        return;
-    }
-
     if ((T == Undef) && (Rho == Undef)) {  // new object, try to pick
         Set(TV,Tcrit()*1.1,Vcrit()*1.1);   // "reasonable" starting point
         t_here = T;
@@ -545,12 +483,8 @@ void Substance::set_xy(int ifx, int ify, double X, double Y,
     Xa = fabs(X);
     Ya = fabs(Y);
 
-
     // loop
     do {
-        if (Err) {
-            break;
-        }
         x_here = prop(ifx);
         y_here = prop(ify);
         err_x = fabs(X - x_here);
@@ -616,8 +550,6 @@ void Substance::set_xy(int ifx, int ify, double X, double Y,
         LoopCount++;
         if (LoopCount > 200) {
             throw TPX_Error("Substance::set_xy","no convergence");
-            set_Err(NoConverge);
-            break;
         }
     } while (1);
 }
@@ -713,7 +645,6 @@ void Substance::set_TPp(double Temp, double Pressure)
                 }
                 if (Vmin >= Vmax) {
                     throw TPX_Error("Substance::set_TPp","Vmin >= Vmax");
-                    set_Err(GenError);
                 } else if ((Vmin > 0.0) && (Vmax < Big)) {
                     kbr = 1;
                 }
@@ -746,8 +677,6 @@ void Substance::set_TPp(double Temp, double Pressure)
         v_here += dv;
         if (dv == 0.0) {
             throw TPX_Error("Substance::set_TPp","dv = 0 and no convergence");
-            set_Err(NoConverge);
-            return;
         }
         Set(TV, Temp, v_here);
         LoopCount++;
@@ -756,8 +685,6 @@ void Substance::set_TPp(double Temp, double Pressure)
             throw TPX_Error("Substance::set_TPp",string("no convergence for ")
                             +"P* = "+fp2str(Pressure/Pcrit())+". V* = "
                             +fp2str(v_save/Vcrit()));
-            set_Err(NoConverge);
-            return;
         }
     }
     Set(TV, Temp,v_here);
diff --git a/src/tpx/Water.cpp b/src/tpx/Water.cpp
index 7e33b1e62..8a8c34a7e 100644
--- a/src/tpx/Water.cpp
+++ b/src/tpx/Water.cpp
@@ -1,9 +1,12 @@
 // water
 
 #include "Water.h"
+#include "cantera/base/stringUtils.h"
 #include <math.h>
 #include <string.h>
 
+using namespace Cantera;
+
 namespace tpx
 {
 
@@ -159,7 +162,8 @@ double water::Psat()
 {
     double log, sum=0,P;
     if ((T < Tmn) || (T > Tc)) {
-        set_Err(TempError);    // Error("water::Psat",TempError,T);
+        throw TPX_Error("water::Psat",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     for (int i=1; i<=8; i++) {
         sum += F[i-1]*pow(aww*(T-Tp),double(i-1));    // DGG mod
@@ -189,7 +193,8 @@ double water::ldens()
     double sum=0;
     int i;
     if ((T < Tmn) || (T >= Tc)) {
-        set_Err(TempError);    // Error("water::ldens",TempError,T);
+        throw TPX_Error("water::ldens",
+                        "Temperature out of range. T = " + fp2str(T));
     }
     for (i=0; i<8; i++) {
         sum+=D[i]*pow(1.0 - T/Tc, double(i+1)/3.0);