diff --git a/src/phreeqcpp/GasPhase.h b/src/phreeqcpp/GasPhase.h
index 7dde49d1..5741f10c 100644
--- a/src/phreeqcpp/GasPhase.h
+++ b/src/phreeqcpp/GasPhase.h
@@ -24,7 +24,8 @@ class cxxGasPhase:public cxxNumKeyword
 	enum GP_TYPE
 	{
 		GP_PRESSURE = 0,
-		GP_VOLUME = 1
+		GP_VOLUME = 1,
+		GP_UNKNOWN = 2
 	};
 
 	//void dump_xml(std::ostream& os, unsigned int indent = 0)const;
diff --git a/src/phreeqcpp/Phreeqc.cpp b/src/phreeqcpp/Phreeqc.cpp
index 604f7b0e..508e1d1b 100644
--- a/src/phreeqcpp/Phreeqc.cpp
+++ b/src/phreeqcpp/Phreeqc.cpp
@@ -453,6 +453,7 @@ void Phreeqc::init(void)
 	last_model.count_kinetics       = -1;
 	last_model.kinetics             = NULL;
 	last_model.count_gas_phase      = -1;
+	last_model.gas_phase_type       = cxxGasPhase::GP_UNKNOWN;
 	last_model.gas_phase            = NULL;
 	last_model.count_ss_assemblage  = -1;
 	last_model.ss_assemblage        = NULL;
diff --git a/src/phreeqcpp/global_structures.h b/src/phreeqcpp/global_structures.h
index aec2ab56..90cb786d 100644
--- a/src/phreeqcpp/global_structures.h
+++ b/src/phreeqcpp/global_structures.h
@@ -1,6 +1,7 @@
 #ifndef _INC_GLOBAL_STRUCTURES_H
 #define _INC_GLOBAL_STRUCTURES_H
 #include "Surface.h"
+#include "GasPhase.h"
 /* ----------------------------------------------------------------------
  *   #define DEFINITIONS
  * ---------------------------------------------------------------------- */
@@ -246,6 +247,7 @@ struct model
 	struct kinetics *kinetics;
 
 	int count_gas_phase;
+	cxxGasPhase::GP_TYPE gas_phase_type;
 	struct phase **gas_phase;
 
 	int count_ss_assemblage;
diff --git a/src/phreeqcpp/prep.cpp b/src/phreeqcpp/prep.cpp
index e565aea6..8deef0a1 100644
--- a/src/phreeqcpp/prep.cpp
+++ b/src/phreeqcpp/prep.cpp
@@ -5860,6 +5860,7 @@ save_model(void)
 	{
 		cxxGasPhase * gas_phase_ptr = use.Get_gas_phase_ptr();
 		last_model.count_gas_phase = (int) gas_phase_ptr->Get_gas_comps().size();
+		last_model.gas_phase_type = gas_phase_ptr->Get_type();
 		last_model.gas_phase =
 			(struct phase **) PHRQ_malloc((size_t) last_model.count_gas_phase *
 										  sizeof(struct phase *));
@@ -5877,6 +5878,7 @@ save_model(void)
 	else
 	{
 		last_model.count_gas_phase = 0;
+		last_model.gas_phase_type = cxxGasPhase::GP_UNKNOWN;
 		last_model.gas_phase = NULL;
 	}
 /*
@@ -6059,6 +6061,8 @@ check_same_model(void)
 			return (FALSE);
 		if (last_model.count_gas_phase != (int) gas_phase_ptr->Get_gas_comps().size())
 			return (FALSE);
+		if (last_model.gas_phase_type != gas_phase_ptr->Get_type())
+			return (FALSE);
 		for (i = 0; i < (int) gas_phase_ptr->Get_gas_comps().size(); i++)
 		{
 			cxxGasComp *gc_ptr = &(gas_phase_ptr->Get_gas_comps()[i]);
diff --git a/src/phreeqcpp/tidy.cpp b/src/phreeqcpp/tidy.cpp
index d953910d..865f01c9 100644
--- a/src/phreeqcpp/tidy.cpp
+++ b/src/phreeqcpp/tidy.cpp
@@ -5471,6 +5471,7 @@ reset_last_model(void)
 	last_model.exchange =
 		(struct master **) free_check_null(last_model.exchange);
 	last_model.count_gas_phase = 0;
+	last_model.gas_phase_type = cxxGasPhase::GP_UNKNOWN;
 	last_model.gas_phase =
 		(struct phase **) free_check_null(last_model.gas_phase);
 	last_model.count_ss_assemblage = 0;