diff --git a/ext/tpx/RedlichKwong.cpp b/ext/tpx/RedlichKwong.cpp
new file mode 100644
index 000000000..17121f60b
--- /dev/null
+++ b/ext/tpx/RedlichKwong.cpp
@@ -0,0 +1,87 @@
+// Lee-Kesler equation of state
+
+#include "RedlichKwong.h"
+#include <math.h>
+
+namespace tpx {
+
+
+    //--------------------------- member functions ------------------
+
+    double RedlichKwong::up() {
+	double u = -Pp()/Rho + hresid() + m_energy_offset;
+        //cout << "up = " << u << endl;
+        return u;
+    }
+
+    double RedlichKwong::hresid(){
+        double hh = m_b * (Rho/m_mw);
+	double hresid_mol_RT =  z() - 1.0 
+                             - (1.5*m_a/(m_b*8314.3*pow(T,1.5)))*log(1.0 + hh);
+        return 8314.3*T*hresid_mol_RT/m_mw;
+    }
+
+    double RedlichKwong::sresid(){
+        double hh = m_b * (Rho/m_mw);
+        //cout << "hh = " << hh << endl;
+	double sresid_mol_R =  log(z()*(1.0 - hh)) 
+                             - (0.5*m_a/(m_b*8314.3*pow(T,1.5)))*log(1.0 + hh);
+        double sp = 8314.3*sresid_mol_R/m_mw;
+        //cout << "sresid = " << sp << endl;
+        return sp;
+    }
+
+    double RedlichKwong::sp() {
+	const double Pref = 101325.0;
+	double rgas = 8314.3/m_mw;
+        //cout << "P = " << Rho*rgas*T << endl;
+        double ss = rgas*(log(Pref/(Rho*rgas*T)));
+        double sr = sresid(); 
+        double p = Pp();
+	double s = rgas*(log(Pref/p)) + sr + m_entropy_offset;
+        //cout << "sp = " << s << " " << ss << " " << sr << " " << m_entropy_offset << endl;
+        return s;
+    }
+
+    double RedlichKwong::z() {
+        return Pp()*m_mw/(Rho*8314.3*T);
+    }
+
+
+    double RedlichKwong::Pp() {
+	double R = 8314.3;
+        double V = m_mw/Rho;
+        double pp = R*T/(V - m_b) - m_a/(sqrt(T)*V*(V+m_b));
+        //cout << "molar V, T, P = " << V << " " << T << " " << pp << endl;
+        return pp;
+        //cout << "Rho, T, Pp = " << pp << endl;
+    }
+
+    double RedlichKwong::Psat(){
+	double tt = m_tcrit/T;
+        double lpr = -0.8734*tt*tt - 3.4522*tt + 4.2918;
+	return m_pcrit*exp(lpr);
+    }
+
+    double RedlichKwong::ldens(){
+        double c;
+        int i;
+        double sqt = sqrt(T);
+        double v = m_b, vnew;
+        double pp = Psat();
+        double Rhsave = Rho;
+        for (i = 0; i < 50; i++) {
+            //pp = Pp();
+            c = m_b*m_b + m_b*GasConstant*T/pp - m_a/(pp*sqt);
+            vnew = (1.0/c)*(v*v*v - GasConstant*T*v*v/pp - m_a*m_b/(pp*sqt));
+            v = vnew;
+            //Rho = m_mw/v;
+            //cout << "ldens Rho = " << Rho << "  " << z() << "  " << Pp() << endl;
+        }
+        Rho = Rhsave;
+        //cout << "ldens: " << m_mw/vnew << endl;
+        return m_mw/vnew;
+    }
+
+
+}
diff --git a/ext/tpx/RedlichKwong.h b/ext/tpx/RedlichKwong.h
new file mode 100644
index 000000000..6f1162659
--- /dev/null
+++ b/ext/tpx/RedlichKwong.h
@@ -0,0 +1,71 @@
+#ifndef TPX_RK_H
+#define TPX_RK_H
+
+#include "Sub.h"
+
+namespace tpx {
+
+    const double GasConstant = 8314.3;
+
+    class RedlichKwong : public Substance{
+
+    public:
+
+	RedlichKwong() : Substance() {
+            setParameters(1.0, 1.0, 1.0);
+        }
+
+        void setParameters(double Tc, double Pc, double MolWt) {
+            m_tcrit = Tc;
+            m_pcrit = Pc;
+            m_mw = MolWt;
+
+            // compute the a and b parameters
+            m_a = 0.42748*GasConstant*GasConstant*pow(m_tcrit, 2.5)/m_pcrit;
+            m_b = 0.08664*GasConstant*m_tcrit/m_pcrit;
+	}
+
+        double a() { return m_a; }
+        double b() { return m_b; }
+
+        // destructor
+	~RedlichKwong() {}
+
+	double MolWt() {return m_mw;}
+	double Tcrit() {return m_tcrit;}
+        double Pcrit() {return m_pcrit;}
+	double Vcrit() {return 0.3592725*GasConstant*T/(m_mw*m_pcrit);}
+	double Tmin() {return 0.0;}
+	double Tmax() {return 1.0e10;}
+	char * name() {return "Redlich-Kwong";}
+	char * formula() {return "-";}
+	
+	double Pp();
+	double up();
+	double sp();
+	double Psat();
+        double dPsatdT(); 
+
+        // compressibility 
+	double z();
+
+        // enthalpy departure
+	double hresid();
+
+        // entropy departure 
+	double sresid();
+
+	double ldens();
+
+    protected:
+
+	double m_tcrit, m_pcrit, m_mw, m_a, m_b;
+        //double m_tmin, m_tmax;
+        //string m_name, m_formula;
+
+    private:
+
+    };
+}
+
+#endif // ! TPX_RK_H