diff --git a/include/cantera/thermo/SpeciesThermoFactory.h b/include/cantera/thermo/SpeciesThermoFactory.h
index 3846f8fa5..80fb0e05d 100644
--- a/include/cantera/thermo/SpeciesThermoFactory.h
+++ b/include/cantera/thermo/SpeciesThermoFactory.h
@@ -108,7 +108,7 @@ public:
      *  @return       Returns the pointer to the newly malloced species
      *                property manager for the reference state
      */
-    SpeciesThermo* newSpeciesThermoManager(std::string& stype) const;
+    SpeciesThermo* newSpeciesThermoManager(const std::string& stype) const;
 
     //! Create a new species property manager for the reference
     //! state for a group of species
@@ -216,7 +216,7 @@ SpeciesThermo* newSpeciesThermoMgr(int type, SpeciesThermoFactory* f=0);
  * @param f           Pointer to a SpeciesThermoFactory. optional parameter.
  *                    Defaults to NULL.
  */
-SpeciesThermo* newSpeciesThermoMgr(std::string& stype,
+SpeciesThermo* newSpeciesThermoMgr(const std::string& stype,
                                    SpeciesThermoFactory* f=0);
 
 //! Function to return SpeciesThermo manager
diff --git a/src/thermo/SpeciesThermoFactory.cpp b/src/thermo/SpeciesThermoFactory.cpp
index 15a289eba..5d685950a 100644
--- a/src/thermo/SpeciesThermoFactory.cpp
+++ b/src/thermo/SpeciesThermoFactory.cpp
@@ -160,7 +160,7 @@ SpeciesThermo* SpeciesThermoFactory::newSpeciesThermo(int type) const
     }
 }
 
-SpeciesThermo* SpeciesThermoFactory::newSpeciesThermoManager(std::string& stype) const
+SpeciesThermo* SpeciesThermoFactory::newSpeciesThermoManager(const std::string& stype) const
 {
     std::string ltype = lowercase(stype);
     if (ltype == "nasa") {
@@ -775,7 +775,7 @@ SpeciesThermo* newSpeciesThermoMgr(int type, SpeciesThermoFactory* f)
     return f->newSpeciesThermo(type);
 }
 
-SpeciesThermo* newSpeciesThermoMgr(std::string& stype,
+SpeciesThermo* newSpeciesThermoMgr(const std::string& stype,
                                    SpeciesThermoFactory* f)
 {
     if (f == 0) {