diff --git a/include/cantera/thermo/GeneralSpeciesThermo.h b/include/cantera/thermo/GeneralSpeciesThermo.h
index df51e4ece..8c945cfed 100644
--- a/include/cantera/thermo/GeneralSpeciesThermo.h
+++ b/include/cantera/thermo/GeneralSpeciesThermo.h
@@ -123,9 +123,9 @@ public:
                               doublereal& maxTemp,
                               doublereal& refPressure) const;
 
-    virtual doublereal reportOneHf298(int k) const;
+    virtual doublereal reportOneHf298(const size_t k) const;
 
-    virtual void modifyOneHf298(const int k, const doublereal Hf298New);
+    virtual void modifyOneHf298(const size_t k, const doublereal Hf298New);
 
 private:
     //! Provide the SpeciesthermoInterpType object
diff --git a/include/cantera/thermo/IdealGasPhase.h b/include/cantera/thermo/IdealGasPhase.h
index 0d0523a3e..c6b91b48a 100644
--- a/include/cantera/thermo/IdealGasPhase.h
+++ b/include/cantera/thermo/IdealGasPhase.h
@@ -768,7 +768,7 @@ public:
      */
     virtual void getEnthalpy_RT_ref(doublereal* hrt) const;
 
-    virtual void modifyOneHf298SS(const size_t& k, const doublereal Hf298New) {
+    virtual void modifyOneHf298SS(const size_t k, const doublereal Hf298New) {
         m_spthermo->modifyOneHf298(k, Hf298New);
         m_tlast += 0.0001234;
     }
diff --git a/include/cantera/thermo/LatticePhase.h b/include/cantera/thermo/LatticePhase.h
index 8818446e8..0ef4a3d23 100644
--- a/include/cantera/thermo/LatticePhase.h
+++ b/include/cantera/thermo/LatticePhase.h
@@ -741,7 +741,7 @@ public:
      *   @param  k           Species k
      *   @param  Hf298New    Specify the new value of the Heat of Formation at 298K and 1 bar
      */
-    virtual void modifyOneHf298SS(const size_t& k, const doublereal Hf298New) {
+    virtual void modifyOneHf298SS(const size_t k, const doublereal Hf298New) {
         m_spthermo->modifyOneHf298(k, Hf298New);
         m_tlast += 0.0001234;
     }
diff --git a/include/cantera/thermo/LatticeSolidPhase.h b/include/cantera/thermo/LatticeSolidPhase.h
index d8d380604..ce7dd21fd 100644
--- a/include/cantera/thermo/LatticeSolidPhase.h
+++ b/include/cantera/thermo/LatticeSolidPhase.h
@@ -632,7 +632,7 @@ public:
      *   @param  k           Species k
      *   @param  Hf298New    Specify the new value of the Heat of Formation at 298K and 1 bar
      */
-    virtual void modifyOneHf298SS(const size_t& k, const doublereal Hf298New);
+    virtual void modifyOneHf298SS(const size_t k, const doublereal Hf298New);
 
 private:
     //! error routine
diff --git a/include/cantera/thermo/MixtureFugacityTP.h b/include/cantera/thermo/MixtureFugacityTP.h
index f149927ce..e046c7365 100644
--- a/include/cantera/thermo/MixtureFugacityTP.h
+++ b/include/cantera/thermo/MixtureFugacityTP.h
@@ -486,7 +486,7 @@ public:
      *   @param  Hf298New    Specify the new value of the Heat of Formation at 298K and 1 bar.
      *                       units = J/kmol.
      */
-    void modifyOneHf298SS(const int k, const doublereal Hf298New);
+    void modifyOneHf298SS(const size_t k, const doublereal Hf298New);
 
     //!  Returns the vector of nondimensional
     //!  Gibbs free energies of the reference state at the current temperature
diff --git a/include/cantera/thermo/NasaPoly1.h b/include/cantera/thermo/NasaPoly1.h
index c6e550e08..760e03575 100644
--- a/include/cantera/thermo/NasaPoly1.h
+++ b/include/cantera/thermo/NasaPoly1.h
@@ -216,7 +216,7 @@ public:
         return h;
     }
 
-    virtual void modifyOneHf298(const size_t& k, const doublereal Hf298New) {
+    virtual void modifyOneHf298(const size_t k, const doublereal Hf298New) {
         if (k != m_index) {
             return;
         }
diff --git a/include/cantera/thermo/SimpleThermo.h b/include/cantera/thermo/SimpleThermo.h
index 626d6dadb..d490f4756 100644
--- a/include/cantera/thermo/SimpleThermo.h
+++ b/include/cantera/thermo/SimpleThermo.h
@@ -263,11 +263,11 @@ public:
         }
     }
 
-    virtual doublereal reportOneHf298(int k) const {
+    virtual doublereal reportOneHf298(const size_t k) const {
         throw CanteraError("reportHF298", "unimplemented");
     }
 
-    virtual void modifyOneHf298(const int k, const doublereal Hf298New) {
+    virtual void modifyOneHf298(const size_t k, const doublereal Hf298New) {
         throw CanteraError("reportHF298", "unimplemented");
     }
 
diff --git a/include/cantera/thermo/SingleSpeciesTP.h b/include/cantera/thermo/SingleSpeciesTP.h
index 1cee0413b..bd0c89aa3 100644
--- a/include/cantera/thermo/SingleSpeciesTP.h
+++ b/include/cantera/thermo/SingleSpeciesTP.h
@@ -334,7 +334,7 @@ public:
      *   @param  k           Species k
      *   @param  Hf298New    Specify the new value of the Heat of Formation at 298K and 1 bar
      */
-    virtual void modifyOneHf298SS(const size_t& k, const doublereal Hf298New) {
+    virtual void modifyOneHf298SS(const size_t k, const doublereal Hf298New) {
         m_spthermo->modifyOneHf298(k, Hf298New);
         m_tlast += 0.0001234;
     }
diff --git a/include/cantera/thermo/SpeciesThermo.h b/include/cantera/thermo/SpeciesThermo.h
index 05a39a8fe..4a1a99d0e 100644
--- a/include/cantera/thermo/SpeciesThermo.h
+++ b/include/cantera/thermo/SpeciesThermo.h
@@ -317,7 +317,7 @@ public:
      *   @param k    species index
      *   @return     Returns the current value of the Heat of Formation at 298K and 1 bar
      */
-    virtual doublereal reportOneHf298(int k) const = 0;
+    virtual doublereal reportOneHf298(const size_t k) const = 0;
 
     //!  Modify the value of the 298 K Heat of Formation of the standard state of
     //!  one species in the phase (J kmol-1)
@@ -329,7 +329,7 @@ public:
      *   @param  Hf298New    Specify the new value of the Heat of Formation at 298K and 1 bar.
      *                       units = J/kmol.
      */
-    virtual void modifyOneHf298(const int k, const doublereal Hf298New) = 0;
+    virtual void modifyOneHf298(const size_t k, const doublereal Hf298New) = 0;
 
 };
 //@}
diff --git a/include/cantera/thermo/SpeciesThermoInterpType.h b/include/cantera/thermo/SpeciesThermoInterpType.h
index 090f40400..82b3d7691 100644
--- a/include/cantera/thermo/SpeciesThermoInterpType.h
+++ b/include/cantera/thermo/SpeciesThermoInterpType.h
@@ -286,7 +286,7 @@ public:
      *   @param  k           Species k
      *   @param  Hf298New    Specify the new value of the Heat of Formation at 298K and 1 bar
      */
-    virtual void modifyOneHf298(const int k, const doublereal Hf298New);
+    virtual void modifyOneHf298(const size_t k, const doublereal Hf298New);
 
 protected:
     //!  lowest valid temperature
diff --git a/include/cantera/thermo/SpeciesThermoMgr.h b/include/cantera/thermo/SpeciesThermoMgr.h
index cbd262ba0..8d9e1e6f9 100644
--- a/include/cantera/thermo/SpeciesThermoMgr.h
+++ b/include/cantera/thermo/SpeciesThermoMgr.h
@@ -107,11 +107,11 @@ public:
                               doublereal& maxTemp,
                               doublereal& refPressure) const;
 
-    virtual doublereal reportOneHf298(int k) const {
+    virtual doublereal reportOneHf298(const size_t k) const {
         throw CanteraError("reportHF298", "unimplemented");
     }
 
-    virtual void modifyOneHf298(const int k, const doublereal Hf298New) {
+    virtual void modifyOneHf298(const size_t k, const doublereal Hf298New) {
         throw CanteraError("reportHF298", "unimplemented");
     }
 
diff --git a/include/cantera/thermo/StoichSubstance.h b/include/cantera/thermo/StoichSubstance.h
index 4e0fdf0e8..aa8a7ca29 100644
--- a/include/cantera/thermo/StoichSubstance.h
+++ b/include/cantera/thermo/StoichSubstance.h
@@ -297,7 +297,7 @@ public:
      */
     virtual void getEnthalpy_RT_ref(doublereal* hrt) const;
 
-    virtual void modifyOneHf298SS(const size_t& k, const doublereal Hf298New) {
+    virtual void modifyOneHf298SS(const size_t k, const doublereal Hf298New) {
         m_spthermo->modifyOneHf298(k, Hf298New);
         m_tlast += 0.0001234;
     }
diff --git a/include/cantera/thermo/SurfPhase.h b/include/cantera/thermo/SurfPhase.h
index 3a80a3f1c..3894f791b 100644
--- a/include/cantera/thermo/SurfPhase.h
+++ b/include/cantera/thermo/SurfPhase.h
@@ -519,7 +519,7 @@ public:
      *   @param  k           Species k
      *   @param  Hf298New    Specify the new value of the Heat of Formation at 298K and 1 bar
      */
-    virtual void modifyOneHf298SS(const size_t& k, const doublereal Hf298New) {
+    virtual void modifyOneHf298SS(const size_t k, const doublereal Hf298New) {
         m_spthermo->modifyOneHf298(k, Hf298New);
         m_tlast += 0.0001234;
     }
diff --git a/include/cantera/thermo/ThermoPhase.h b/include/cantera/thermo/ThermoPhase.h
index 12ac4ddbd..a163a682e 100644
--- a/include/cantera/thermo/ThermoPhase.h
+++ b/include/cantera/thermo/ThermoPhase.h
@@ -189,7 +189,7 @@ public:
      *   @param  k           Species k
      *   @param  Hf298New    Specify the new value of the Heat of Formation at 298K and 1 bar
      */
-    virtual void modifyOneHf298SS(const size_t& k, const doublereal Hf298New) {
+    virtual void modifyOneHf298SS(const size_t k, const doublereal Hf298New) {
         m_spthermo->modifyOneHf298(k, Hf298New);
     }
 
diff --git a/include/cantera/thermo/VPStandardStateTP.h b/include/cantera/thermo/VPStandardStateTP.h
index 79a7f525a..96bfb34c3 100644
--- a/include/cantera/thermo/VPStandardStateTP.h
+++ b/include/cantera/thermo/VPStandardStateTP.h
@@ -407,7 +407,7 @@ public:
      *   @param  Hf298New    Specify the new value of the Heat of Formation at 298K and 1 bar.
      *                       units = J/kmol.
      */
-    void modifyOneHf298SS(const size_t& k, const doublereal Hf298New);
+    void modifyOneHf298SS(const size_t k, const doublereal Hf298New);
 
     //!  Returns the vector of nondimensional
     //!  Gibbs free energies of the reference state at the current temperature
diff --git a/src/thermo/ConstCpPoly.cpp b/src/thermo/ConstCpPoly.cpp
index e3c9d762d..d91d5a5e0 100644
--- a/src/thermo/ConstCpPoly.cpp
+++ b/src/thermo/ConstCpPoly.cpp
@@ -120,7 +120,7 @@ doublereal ConstCpPoly::reportHf298(doublereal* const h298) const
     return h;
 }
 
-void ConstCpPoly::modifyOneHf298(const size_t& k, const doublereal Hf298New)
+void ConstCpPoly::modifyOneHf298(const size_t k, const doublereal Hf298New)
 {
     if (k != m_index) {
         return;
diff --git a/src/thermo/ConstCpPoly.h b/src/thermo/ConstCpPoly.h
index d5a7d28cd..b9e27c3dc 100644
--- a/src/thermo/ConstCpPoly.h
+++ b/src/thermo/ConstCpPoly.h
@@ -115,7 +115,7 @@ public:
     virtual void modifyParameters(doublereal* coeffs);
 
     virtual doublereal reportHf298(doublereal* const h298 = 0) const;
-    virtual void modifyOneHf298(const size_t& k, const doublereal Hf298New);
+    virtual void modifyOneHf298(const size_t k, const doublereal Hf298New);
 
 protected:
     //! Base temperature
diff --git a/src/thermo/GeneralSpeciesThermo.cpp b/src/thermo/GeneralSpeciesThermo.cpp
index 05b30c75a..098cba653 100644
--- a/src/thermo/GeneralSpeciesThermo.cpp
+++ b/src/thermo/GeneralSpeciesThermo.cpp
@@ -304,7 +304,7 @@ SpeciesThermoInterpType* GeneralSpeciesThermo::provideSTIT(size_t k)
     return m_sp[k];
 }
 
-doublereal GeneralSpeciesThermo::reportOneHf298(int k) const
+doublereal GeneralSpeciesThermo::reportOneHf298(const size_t k) const
 {
     SpeciesThermoInterpType* sp_ptr = m_sp[k];
     doublereal h = -1.0;
@@ -314,7 +314,7 @@ doublereal GeneralSpeciesThermo::reportOneHf298(int k) const
     return h;
 }
 
-void GeneralSpeciesThermo::modifyOneHf298(const int k, const doublereal Hf298New)
+void GeneralSpeciesThermo::modifyOneHf298(const size_t k, const doublereal Hf298New)
 {
     SpeciesThermoInterpType* sp_ptr = m_sp[k];
     if (sp_ptr) {
diff --git a/src/thermo/LatticeSolidPhase.cpp b/src/thermo/LatticeSolidPhase.cpp
index 032bff74b..29e9bd143 100644
--- a/src/thermo/LatticeSolidPhase.cpp
+++ b/src/thermo/LatticeSolidPhase.cpp
@@ -512,7 +512,7 @@ void LatticeSolidPhase::setParametersFromXML(const XML_Node& eosdata)
 
 }
 
-void LatticeSolidPhase::modifyOneHf298SS(const size_t& k, const doublereal Hf298New)
+void LatticeSolidPhase::modifyOneHf298SS(const size_t k, const doublereal Hf298New)
 {
     for (size_t n = 0; n < m_nlattice; n++) {
         if (lkstart_[n+1] < k) {
diff --git a/src/thermo/MixtureFugacityTP.cpp b/src/thermo/MixtureFugacityTP.cpp
index 3916c5624..dcbd5601e 100644
--- a/src/thermo/MixtureFugacityTP.cpp
+++ b/src/thermo/MixtureFugacityTP.cpp
@@ -147,7 +147,7 @@ void MixtureFugacityTP::getEnthalpy_RT(doublereal* hrt) const
     getEnthalpy_RT_ref(hrt);
 }
 
-void MixtureFugacityTP::modifyOneHf298SS(const int k, const doublereal Hf298New)
+void MixtureFugacityTP::modifyOneHf298SS(const size_t k, const doublereal Hf298New)
 {
     m_spthermo->modifyOneHf298(k, Hf298New);
     m_Tlast_ref += 0.0001234;
diff --git a/src/thermo/NasaPoly2.h b/src/thermo/NasaPoly2.h
index 68fa557a1..e9bbe093e 100644
--- a/src/thermo/NasaPoly2.h
+++ b/src/thermo/NasaPoly2.h
@@ -178,7 +178,7 @@ public:
         return h;
     }
 
-    void modifyOneHf298(const size_t& k, const doublereal Hf298New) {
+    void modifyOneHf298(const size_t k, const doublereal Hf298New) {
         if (k != m_index) {
             return;
         }
diff --git a/src/thermo/NasaThermo.cpp b/src/thermo/NasaThermo.cpp
index ea87cf185..5a3d3e720 100644
--- a/src/thermo/NasaThermo.cpp
+++ b/src/thermo/NasaThermo.cpp
@@ -209,10 +209,10 @@ void NasaThermo::reportParams(size_t index, int& type,
     }
 }
 
-doublereal NasaThermo::reportOneHf298(const int k) const
+doublereal NasaThermo::reportOneHf298(const size_t k) const
 {
-    int grp = m_group_map[k];
-    int pos = m_posInGroup_map[k];
+    size_t grp = m_group_map[k];
+    size_t pos = m_posInGroup_map[k];
     const std::vector<NasaPoly1> &mlg = m_low[grp-1];
     const NasaPoly1* nlow = &(mlg[pos]);
     doublereal tmid = nlow->maxTemp();
@@ -227,10 +227,10 @@ doublereal NasaThermo::reportOneHf298(const int k) const
     return h;
 }
 
-void NasaThermo::modifyOneHf298(const int k, const doublereal Hf298New)
+void NasaThermo::modifyOneHf298(const size_t k, const doublereal Hf298New)
 {
-    int grp = m_group_map[k];
-    int pos = m_posInGroup_map[k];
+    size_t grp = m_group_map[k];
+    size_t pos = m_posInGroup_map[k];
     std::vector<NasaPoly1> &mlg = m_low[grp-1];
     NasaPoly1* nlow = &(mlg[pos]);
     std::vector<NasaPoly1> &mhg = m_high[grp-1];
diff --git a/src/thermo/NasaThermo.h b/src/thermo/NasaThermo.h
index 62ddaab2e..794703adc 100644
--- a/src/thermo/NasaThermo.h
+++ b/src/thermo/NasaThermo.h
@@ -145,8 +145,8 @@ public:
                               doublereal& maxTemp,
                               doublereal& refPressure) const;
 
-    virtual doublereal reportOneHf298(const int k) const;
-    virtual void modifyOneHf298(const int k, const doublereal Hf298New);
+    virtual doublereal reportOneHf298(const size_t k) const;
+    virtual void modifyOneHf298(const size_t k, const doublereal Hf298New);
 
     //! Initialized to the type of parameterization
     /*!
diff --git a/src/thermo/ShomatePoly.h b/src/thermo/ShomatePoly.h
index bb0d840be..414e3c309 100644
--- a/src/thermo/ShomatePoly.h
+++ b/src/thermo/ShomatePoly.h
@@ -243,7 +243,7 @@ public:
         return hh;
     }
 
-    virtual void modifyOneHf298(const int k, const doublereal Hf298New) {
+    virtual void modifyOneHf298(const size_t k, const doublereal Hf298New) {
         doublereal hnow = reportHf298();
         doublereal delH = Hf298New - hnow;
         m_coeff[5] += delH / 1.0E6;
@@ -476,7 +476,7 @@ public:
         return h;
     }
 
-    virtual void modifyOneHf298(const size_t& k, const doublereal Hf298New) {
+    virtual void modifyOneHf298(const size_t k, const doublereal Hf298New) {
         if (k != m_index) {
             return;
         }
diff --git a/src/thermo/ShomateThermo.h b/src/thermo/ShomateThermo.h
index 4a7942044..693c9c2ee 100644
--- a/src/thermo/ShomateThermo.h
+++ b/src/thermo/ShomateThermo.h
@@ -328,12 +328,12 @@ public:
         }
     }
 
-    virtual doublereal reportOneHf298(int k) const {
+    virtual doublereal reportOneHf298(const size_t k) const {
         doublereal h;
         doublereal t = 298.15;
 
-        int grp = m_group_map[k];
-        int pos = m_posInGroup_map[k];
+        size_t grp = m_group_map[k];
+        size_t pos = m_posInGroup_map[k];
         const std::vector<ShomatePoly> &mlg = m_low[grp-1];
         const ShomatePoly* nlow = &(mlg[pos]);
 
@@ -348,10 +348,10 @@ public:
         return h;
     }
 
-    virtual void modifyOneHf298(const int k, const doublereal Hf298New) {
+    virtual void modifyOneHf298(const size_t k, const doublereal Hf298New) {
 
-        int grp = m_group_map[k];
-        int pos = m_posInGroup_map[k];
+        size_t grp = m_group_map[k];
+        size_t pos = m_posInGroup_map[k];
         std::vector<ShomatePoly> &mlg = m_low[grp-1];
         ShomatePoly* nlow = &(mlg[pos]);
         std::vector<ShomatePoly> &mhg = m_high[grp-1];
diff --git a/src/thermo/SpeciesThermoInterpType.cpp b/src/thermo/SpeciesThermoInterpType.cpp
index 4b8bf38cb..e3092e5c5 100644
--- a/src/thermo/SpeciesThermoInterpType.cpp
+++ b/src/thermo/SpeciesThermoInterpType.cpp
@@ -57,7 +57,7 @@ doublereal SpeciesThermoInterpType::reportHf298(doublereal* const h298) const
 }
 //====================================================================================================
 
-void SpeciesThermoInterpType::modifyOneHf298(const int k, const doublereal Hf298New)
+void SpeciesThermoInterpType::modifyOneHf298(const size_t k, const doublereal Hf298New)
 {
     throw CanteraError("SpeciesThermoInterpType::modifyOneHf298",
                        "Not implemented");
diff --git a/src/thermo/VPStandardStateTP.cpp b/src/thermo/VPStandardStateTP.cpp
index b4c4794de..d48704e96 100644
--- a/src/thermo/VPStandardStateTP.cpp
+++ b/src/thermo/VPStandardStateTP.cpp
@@ -162,7 +162,7 @@ void VPStandardStateTP::getEnthalpy_RT(doublereal* hrt) const
 }
 
 //================================================================================================
-void VPStandardStateTP::modifyOneHf298SS(const size_t& k, const doublereal Hf298New)
+void VPStandardStateTP::modifyOneHf298SS(const size_t k, const doublereal Hf298New)
 {
     m_spthermo->modifyOneHf298(k, Hf298New);
     m_Tlast_ss += 0.0001234;