diff --git a/Phreeqc.cpp b/Phreeqc.cpp
index 54bf7f4b..4236a163 100644
--- a/Phreeqc.cpp
+++ b/Phreeqc.cpp
@@ -978,9 +978,9 @@ void Phreeqc::init(void)
 	/* ----------------------------------------------------------------------
 	*   ISOTOPES
 	* ---------------------------------------------------------------------- */
-	count_master_isotope	= 0;
-	master_isotope			= NULL;
-	max_master_isotope		= MAX_ELTS;
+	//count_master_isotope	= 0;
+	//master_isotope			= NULL;
+	//max_master_isotope		= MAX_ELTS;
 	initial_solution_isotopes = FALSE;
 	calculate_value_hash_table = NULL;	
 	count_isotope_ratio		= 0;
@@ -2277,7 +2277,7 @@ Phreeqc::InternalCopy(const Phreeqc *pSrc)
 	max_master_isotope		= MAX_ELTS;
 	*/
 
-	for (int i = 0; i < pSrc->count_master_isotope; i++)
+	for (int i = 0; i < (int)pSrc->master_isotope.size(); i++)
 	{
 		struct master_isotope *master_isotope_ptr = master_isotope_store(pSrc->master_isotope[i]->name, FALSE);
 		memcpy(master_isotope_ptr, pSrc->master_isotope[i], sizeof(struct master_isotope));
diff --git a/Phreeqc.h b/Phreeqc.h
index 6562717e..a261d851 100644
--- a/Phreeqc.h
+++ b/Phreeqc.h
@@ -1727,9 +1727,10 @@ protected:
 	* ---------------------------------------------------------------------- */
 	//struct name_coef match_tokens[50];
 	//int count_match_tokens;
-	int count_master_isotope;
-	struct master_isotope **master_isotope;
-	int max_master_isotope;
+	//int count_master_isotope;
+	//struct master_isotope **master_isotope;
+	//int max_master_isotope;
+	std::vector<master_isotope*> master_isotope;
 	int initial_solution_isotopes;
 	std::vector<struct calculate_value*> calculate_value;
 	HashTable *calculate_value_hash_table;
diff --git a/isotopes.cpp b/isotopes.cpp
index 9e4f28ac..d6f30cba 100644
--- a/isotopes.cpp
+++ b/isotopes.cpp
@@ -478,7 +478,8 @@ add_isotopes(cxxSolution &solution_ref)
 	/*
 	 * zero out isotopes
 	 */
-	for (i = 0; i < count_master_isotope; i++)
+	//for (i = 0; i < count_master_isotope; i++)
+	for (i = 0; i < (int)master_isotope.size(); i++)
 	{
 		master_isotope[i]->moles = 0;
 	}
@@ -513,7 +514,7 @@ add_isotopes(cxxSolution &solution_ref)
 	 * Set isotopes flag
 	 */
 	initial_solution_isotopes = FALSE;
-	for (i = 0; i < count_master_isotope; i++)
+	for (i = 0; i < (int)master_isotope.size(); i++)
 	{
 		if (master_isotope[i]->minor_isotope == TRUE
 			&& master_isotope[i]->moles > 0)
@@ -662,7 +663,7 @@ calculate_isotope_moles(struct element *elt_ptr,
 	/*
 	 *  Update master_isotope
 	 */
-	for (j = 0; j < count_master_isotope; j++)
+	for (j = 0; j < (int)master_isotope.size(); j++)
 	{
 		for (i = 0; i < count_isotopes; i++)
 		{
@@ -770,12 +771,12 @@ print_initial_solution_isotopes(void)
 	print_centered("Isotopes");
 	output_msg(sformatf( "%10s\t%12s\t%12s\t%12s\t%12s\n\n", "Isotope",
 			   "Molality", "Moles", "Ratio", "Units"));
-	for (i = 0; i < count_master_isotope; i++)
+	for (i = 0; i < (int)master_isotope.size(); i++)
 	{
 		if (master_isotope[i]->minor_isotope == FALSE)
 		{
 			print_isotope = FALSE;
-			for (j = 0; j < count_master_isotope; j++)
+			for (j = 0; j < (int)master_isotope.size(); j++)
 			{
 				if ((master_isotope[j]->elt == master_isotope[i]->elt) &&
 					(master_isotope[j]->minor_isotope == TRUE) &&
@@ -794,7 +795,7 @@ print_initial_solution_isotopes(void)
 					   master_isotope[i]->name,
 					   (double) (master_isotope[i]->moles / mass_water_aq_x),
 					   (double) master_isotope[i]->moles));
-			for (j = 0; j < count_master_isotope; j++)
+			for (j = 0; j < (int)master_isotope.size(); j++)
 			{
 				if (i == j)
 					continue;
@@ -991,7 +992,7 @@ print_isotope_ratios(void)
  *   Print heading
  */
 	print_isotope = FALSE;
-	for (i = 0; i < count_master_isotope; i++)
+	for (i = 0; i < (int)master_isotope.size(); i++)
 	{
 		if (master_isotope[i]->minor_isotope == FALSE)
 			continue;
@@ -1053,7 +1054,7 @@ print_isotope_alphas(void)
  *   Print heading
  */
 	print_isotope = FALSE;
-	for (i = 0; i < count_master_isotope; i++)
+	for (i = 0; i < (int)master_isotope.size(); i++)
 	{
 		if (master_isotope[i]->minor_isotope == FALSE)
 			continue;
@@ -1350,13 +1351,15 @@ master_isotope_store(const char *name, int replace_if_found)
 	}
 	else
 	{
-		n = count_master_isotope++;
-		/* make sure there is space in s */
-		if (count_master_isotope >= max_master_isotope)
-		{
-			space((void **) ((void *) &master_isotope), count_master_isotope,
-				  &max_master_isotope, sizeof(struct master_isotope *));
-		}
+		//n = count_master_isotope++;
+		///* make sure there is space in s */
+		//if (count_master_isotope >= max_master_isotope)
+		//{
+		//	space((void **) ((void *) &master_isotope), count_master_isotope,
+		//		  &max_master_isotope, sizeof(struct master_isotope *));
+		//}
+		n = (int)master_isotope.size();
+		master_isotope.resize((size_t)n + 1);
 		/* Make new master_isotope structure */
 		master_isotope[n] = master_isotope_alloc();
 		master_isotope_ptr = master_isotope[n];
diff --git a/mainsubs.cpp b/mainsubs.cpp
index 9df03df9..6e49dddd 100644
--- a/mainsubs.cpp
+++ b/mainsubs.cpp
@@ -98,14 +98,14 @@ initialize(void)
 
 	space((void **) ((void *) &logk), INIT, &max_logk, sizeof(struct logk *));
 
-	space((void **) ((void *) &master_isotope), INIT, &max_master_isotope,
-		  sizeof(struct master_isotope *));
+	//space((void **) ((void *) &master_isotope), INIT, &max_master_isotope,
+	//	  sizeof(struct master_isotope *));
 
 /*
  *   Create hash tables
  */
 	hcreate_multi((unsigned) max_logk, &logk_hash_table);
-	hcreate_multi((unsigned) max_master_isotope, &master_isotope_hash_table);
+	hcreate_multi((unsigned) MAX_ELTS, &master_isotope_hash_table);
 	hcreate_multi((unsigned) max_elements, &elements_hash_table);
 	hcreate_multi((unsigned) max_s, &species_hash_table);
 	hcreate_multi((unsigned) max_phases, &phases_hash_table);
@@ -1379,7 +1379,7 @@ xsolution_save(int n_user)
 	 */
 	if (initial_solution_isotopes == TRUE)
 	{
-		for (int i = 0; i < count_master_isotope; i++)
+		for (int i = 0; i < (int)master_isotope.size(); i++)
 		{
 			if (master_isotope[i]->moles > 0)
 			{
diff --git a/structures.cpp b/structures.cpp
index bef9008e..79d43539 100644
--- a/structures.cpp
+++ b/structures.cpp
@@ -225,13 +225,11 @@ clean_up(void)
 	user_graph_headings = (char **) free_check_null(user_graph_headings);
 #endif
 	/* master_isotope */
-	for (i = 0; i < count_master_isotope; i++)
+	for (i = 0; i < (int)master_isotope.size(); i++)
 	{
-		master_isotope[i] =
-			(struct master_isotope *) free_check_null(master_isotope[i]);
+		master_isotope[i] =	(struct master_isotope *) free_check_null(master_isotope[i]);
 	}
-	master_isotope =
-		(struct master_isotope **) free_check_null(master_isotope);
+	//master_isotope = (struct master_isotope **) free_check_null(master_isotope);
 	hdestroy_multi(master_isotope_hash_table);
 	master_isotope_hash_table = NULL;
 
@@ -320,7 +318,7 @@ clean_up(void)
 	count_phases = 0;
 	count_s = 0;
 	count_logk = 0;
-	count_master_isotope = 0;
+	//count_master_isotope = 0;
 	count_rates = 0;
 	count_inverse = 0;
 	count_save_values = 0;
diff --git a/tidy.cpp b/tidy.cpp
index 665e8a5f..be3f71b3 100644
--- a/tidy.cpp
+++ b/tidy.cpp
@@ -5505,7 +5505,7 @@ tidy_master_isotope(void)
 	int i;
 	struct master *master_ptr;
 
-	for (i = 0; i < count_master_isotope; i++)
+	for (i = 0; i < (int)master_isotope.size(); i++)
 	{
 		/*
 		 * Mark master species list as minor isotope