diff --git a/include/cantera/thermo/AdsorbateThermo.h b/include/cantera/thermo/AdsorbateThermo.h
index 7c02dcf8f..62e38589c 100644
--- a/include/cantera/thermo/AdsorbateThermo.h
+++ b/include/cantera/thermo/AdsorbateThermo.h
@@ -49,7 +49,7 @@ public:
      * @param thigh     output - Maximum temperature
      * @param pref      output - reference pressure (Pa).
      */
-    Adsorbate(int n, doublereal tlow, doublereal thigh, doublereal pref,
+    Adsorbate(size_t n, doublereal tlow, doublereal thigh, doublereal pref,
               const doublereal* coeffs) : m_lowT(tlow),
         m_highT(thigh),
         m_index(n) {
@@ -81,7 +81,7 @@ public:
         return (SpeciesThermoInterpType*) np;
     }
 
-    virtual void install(std::string name, int index, int type,
+    virtual void install(std::string name, size_t index, int type,
                          const doublereal* c, doublereal minTemp, doublereal maxTemp,
                          doublereal refPressure) {
         m_be = c[1];
@@ -174,7 +174,7 @@ public:
         tlow = m_lowT;
         thigh = m_highT;
         pref = m_Pref;
-        coeffs[0] = m_nFreqs;
+        coeffs[0] = static_cast<double>(m_nFreqs);
         coeffs[1] = m_be;
         for (size_t i = 2; i < m_nFreqs+2; i++) {
             coeffs[i] = m_freq[i-2];
@@ -189,7 +189,7 @@ protected:
     //! Reference state pressure
     doublereal m_Pref;
     //! species index
-    int m_index;
+    size_t m_index;
     size_t m_nFreqs;
     //! array of vib frequencies
     vector_fp m_freq;
diff --git a/src/thermo/SpeciesThermoFactory.cpp b/src/thermo/SpeciesThermoFactory.cpp
index 1ad39bb80..04e7ddebb 100644
--- a/src/thermo/SpeciesThermoFactory.cpp
+++ b/src/thermo/SpeciesThermoFactory.cpp
@@ -677,12 +677,12 @@ static void installNasa9ThermoFromXML(std::string speciesName,
  *  @param tp                 Vector of XML Nodes that make up the parameterization
  */
 static void installAdsorbateThermoFromXML(std::string speciesName,
-        SpeciesThermo& sp, int k,
+        SpeciesThermo& sp, size_t k,
         const XML_Node& f)
 {
     vector_fp freqs;
     doublereal tmin, tmax, pref = OneAtm;
-    int nfreq = 0;
+    size_t nfreq = 0;
     tmin = fpValue(f["Tmin"]);
     tmax = fpValue(f["Tmax"]);
     if (f.hasAttrib("P0")) {
@@ -699,11 +699,11 @@ static void installAdsorbateThermoFromXML(std::string speciesName,
         getFloatArray(f.child("floatArray"), freqs, false);
         nfreq = freqs.size();
     }
-    for (int n = 0; n < nfreq; n++) {
+    for (size_t n = 0; n < nfreq; n++) {
         freqs[n] *= 3.0e10;
     }
     vector_fp coeffs(nfreq + 2);
-    coeffs[0] = nfreq;
+    coeffs[0] = static_cast<double>(nfreq);
     coeffs[1] = getFloat(f, "binding_energy", "toSI");
     copy(freqs.begin(), freqs.end(), coeffs.begin() + 2);
     (&sp)->install(speciesName, k, ADSORBATE, &coeffs[0], tmin, tmax, pref);