diff --git a/Cantera/src/zeroD/ConstPressureReactor.cpp b/Cantera/src/zeroD/ConstPressureReactor.cpp
index 6f90a088f..4ca0b984b 100644
--- a/Cantera/src/zeroD/ConstPressureReactor.cpp
+++ b/Cantera/src/zeroD/ConstPressureReactor.cpp
@@ -48,7 +48,7 @@ namespace CanteraZeroD {
         // coverages on the walls
         size_t loc = m_nsp + 2;
         SurfPhase* surf;
-        for (int m = 0; m < m_nwalls; m++) {
+        for (size_t m = 0; m < m_nwalls; m++) {
             surf = m_wall[m]->surface(m_lr[m]);
             if (surf) {
                 m_wall[m]->getCoverages(m_lr[m], y + loc);
@@ -61,7 +61,7 @@ namespace CanteraZeroD {
         m_thermo->restoreState(m_state);
         m_sdot.resize(m_nsp, 0.0);
         m_nv = m_nsp + 2;
-        for (int w = 0; w < m_nwalls; w++)
+        for (size_t w = 0; w < m_nwalls; w++)
             if (m_wall[w]->surface(m_lr[w]))
                 m_nv += m_wall[w]->surface(m_lr[w])->nSpecies();
         m_enthalpy = m_thermo->enthalpy_mass();
@@ -106,7 +106,7 @@ namespace CanteraZeroD {
 
         size_t loc = m_nsp + 2;
         SurfPhase* surf;
-        for (int m = 0; m < m_nwalls; m++) {
+        for (size_t m = 0; m < m_nwalls; m++) {
             surf = m_wall[m]->surface(m_lr[m]);
             if (surf) {
                 m_wall[m]->setCoverages(m_lr[m], y+loc);
diff --git a/Cantera/src/zeroD/Reactor.cpp b/Cantera/src/zeroD/Reactor.cpp
index 2413b516e..1d57c4337 100644
--- a/Cantera/src/zeroD/Reactor.cpp
+++ b/Cantera/src/zeroD/Reactor.cpp
@@ -288,11 +288,10 @@ namespace CanteraZeroD {
 
             // outlets 
 
-            int n;
             doublereal mdot_out;
             for (i = 0; i < m_nOutlets; i++) {
                 mdot_out = m_outlet[i]->massFlowRate(time);
-                for (n = 0; n < m_nsp; n++) {
+                for (size_t n = 0; n < m_nsp; n++) {
                     ydot[2+n] -= mdot_out * mf[n];
                 }
                 if (m_energy) {
@@ -333,13 +332,13 @@ namespace CanteraZeroD {
         }
     }
 
-    void Reactor::addSensitivityReaction(int rxn) {
+    void Reactor::addSensitivityReaction(size_t rxn) {
         m_pnum.push_back(rxn);
         m_pname.push_back(name()+": "+m_kin->reactionString(rxn));
         m_mult_save.push_back(1.0);
-        if (rxn < 0 || rxn >= m_kin->nReactions()) 
+        if (rxn >= m_kin->nReactions())
             throw CanteraError("Reactor::addSensitivityReaction",
-                "Reaction number out of range ("+int2str(rxn)+")");
+                "Reaction number out of range ("+int2str(int(rxn))+")");
     }
 
 
diff --git a/Cantera/src/zeroD/Reactor.h b/Cantera/src/zeroD/Reactor.h
index 7aa66ecc6..4c4ae270d 100644
--- a/Cantera/src/zeroD/Reactor.h
+++ b/Cantera/src/zeroD/Reactor.h
@@ -103,7 +103,7 @@ namespace CanteraZeroD {
         virtual void updateState(doublereal* y);
 
         virtual size_t nSensParams();
-        virtual void addSensitivityReaction(int rxn);
+        virtual void addSensitivityReaction(size_t rxn);
 
         virtual std::string sensParamID(int p) { return m_pname[p]; }
 
diff --git a/Cantera/src/zeroD/ReactorBase.h b/Cantera/src/zeroD/ReactorBase.h
index e84fb96f2..68fcb4025 100644
--- a/Cantera/src/zeroD/ReactorBase.h
+++ b/Cantera/src/zeroD/ReactorBase.h
@@ -156,7 +156,7 @@ namespace CanteraZeroD {
         std::vector<FlowDevice*> m_inlet, m_outlet;
         std::vector<Wall*> m_wall;
         vector_int m_lr;
-        int m_nwalls;
+        size_t m_nwalls;
         std::string m_name;
         double m_rho0;
 
diff --git a/Cantera/src/zeroD/ReactorNet.cpp b/Cantera/src/zeroD/ReactorNet.cpp
index 16c924970..f80ec4e31 100644
--- a/Cantera/src/zeroD/ReactorNet.cpp
+++ b/Cantera/src/zeroD/ReactorNet.cpp
@@ -28,7 +28,7 @@ namespace CanteraZeroD {
   }
 
   ReactorNet::~ReactorNet() {
-    for (int n = 0; n < m_nr; n++) {
+    for (size_t n = 0; n < m_nr; n++) {
       if (m_iown[n]) {
 	delete m_r[n];
       }
@@ -204,7 +204,6 @@ namespace CanteraZeroD {
         
   void ReactorNet::evalJacobian(doublereal t, doublereal* y, 
 				doublereal* ydot, doublereal* p, Array2D* j) {
-    int n, m;
     doublereal ysave, dy;
     Array2D& jac = *j;
 
@@ -213,7 +212,7 @@ namespace CanteraZeroD {
     try {
       //evaluate the unperturbed ydot
       eval(t, y, ydot, p);
-      for (n = 0; n < m_nv; n++) {
+      for (size_t n = 0; n < m_nv; n++) {
              
 	// perturb x(n)
 	ysave = y[n];
@@ -225,7 +224,7 @@ namespace CanteraZeroD {
 	eval(t, y, DATA_PTR(m_ydot), p);
 
 	// compute nth column of Jacobian
-	for (m = 0; m < m_nv; m++) {
+	for (size_t m = 0; m < m_nv; m++) {
 	  jac(m,n) = (m_ydot[m] - ydot[m])/dy;
 	}
 	y[n] = ysave;
diff --git a/Cantera/src/zeroD/Wall.cpp b/Cantera/src/zeroD/Wall.cpp
index 9aebc3823..476f43fa1 100644
--- a/Cantera/src/zeroD/Wall.cpp
+++ b/Cantera/src/zeroD/Wall.cpp
@@ -120,8 +120,8 @@ namespace CanteraZeroD {
             m_surf[1]->setCoverages(DATA_PTR(m_rightcov));
     }
 
-    void Wall::addSensitivityReaction(int leftright, int rxn) {
-        if (rxn < 0 || rxn >= m_chem[leftright]->nReactions()) 
+    void Wall::addSensitivityReaction(int leftright, size_t rxn) {
+        if (rxn >= m_chem[leftright]->nReactions())
             throw CanteraError("Wall::addSensitivityReaction",
                 "Reaction number out of range ("+int2str(rxn)+")");
         if (leftright == 0) {
diff --git a/Cantera/src/zeroD/Wall.h b/Cantera/src/zeroD/Wall.h
index 1aa8199d0..47a7ec8f8 100644
--- a/Cantera/src/zeroD/Wall.h
+++ b/Cantera/src/zeroD/Wall.h
@@ -130,8 +130,8 @@ namespace CanteraZeroD {
             else
                 return m_pright.size();
         }
-        void addSensitivityReaction(int leftright, int rxn);
-        std::string sensitivityParamID(int leftright, int p) { 
+        void addSensitivityReaction(int leftright, size_t rxn);
+        std::string sensitivityParamID(int leftright, size_t p) {
             if (leftright == 0)
                 return m_pname_left[p]; 
             else