diff --git a/Cantera/src/mix_defs.h b/Cantera/src/mix_defs.h
index 4613b0d1a..832e46b82 100755
--- a/Cantera/src/mix_defs.h
+++ b/Cantera/src/mix_defs.h
@@ -3,18 +3,38 @@
 
 namespace Cantera {
 
+    /**
+     *  This generic id is used as the default in virtual base
+     *  classes that employ id's. It is used to indicate the lack
+     *  of an inherited class that would define the id.
+     */
     const int cNone = 0;
 
     // species thermo types
     const int cNASA = 1;
     const int cShomate = 2;
 
-    // equation of state types
-    const int cIdealGas = 1;
-    const int cIncompressible = 2;
-    const int cSurf = 3;
-    const int cMetal = 4;
-    const int cSolidCompound = 5;
+    /**
+     * Equation of state types:
+     *
+     *     These types are used in the member function eosType()
+     *  of the virtual base class ThermoPhase. They are used to
+     *  distinguish different types of equation of states. And,
+     *  they may be used for upcasting from the ThermoPhase class.
+     *  Their id's should be distinct.
+     *
+     *  Users who wish to define their own equation of states which
+     *  inherit from ThermoPhase should define a unique id which
+     *  doesn't conflict with those listed below. The Cantera Kernel
+     *  however, will not be know about the class and will therefore
+     *  not be able to initialize the class within its "factory"
+     *  routines. 
+     */
+    const int cIdealGas = 1;       //  IdealGasPhase in IdealGasPhase.h
+    const int cIncompressible = 2; //  ConstDensityThermo in ConstDensityThermo.h
+    const int cSurf = 3;           //  SurfPhase in SurfPhase.h 
+    const int cMetal = 4;          //  MetalPhase in MetalPhase.h
+    const int cSolidCompound = 5;  //  SolidCompound in SolidCompound.h
 
     // kinetic manager types
     const int cGasKinetics = 2;