diff --git a/Phreeqc.cpp b/Phreeqc.cpp
index 189276a9..7551fe2f 100644
--- a/Phreeqc.cpp
+++ b/Phreeqc.cpp
@@ -904,16 +904,11 @@ void Phreeqc::init(void)
 	remove_unstable_phases  = FALSE;
 	// auto screen_string;
 	spread_length           = 10;
-	/* ---------------------------------------------------------------------- */
-	/*
-	*   Hash definitions
-	*/
-	master_isotope_hash_table = NULL;
 	/* ----------------------------------------------------------------------
 	*   ISOTOPES
 	* ---------------------------------------------------------------------- */
 	initial_solution_isotopes = FALSE;
-	calculate_value_hash_table = NULL;	
+	calculate_value_hash_table = NULL;
 	isotope_ratio_hash_table = 0;	
 	isotope_alpha_hash_table = 0;
 
diff --git a/Phreeqc.h b/Phreeqc.h
index 930b7245..4614233a 100644
--- a/Phreeqc.h
+++ b/Phreeqc.h
@@ -1665,7 +1665,7 @@ protected:
 	std::map<std::string, struct species*> species_map;
 	std::map<std::string, struct phase*> phases_map;
 	std::map<std::string, struct logk*> logk_map;
-	HashTable *master_isotope_hash_table;
+	std::map<std::string, struct master_isotope*> master_isotope_map;
 
 #if defined(PHREEQCI_GUI)
 #include "../../phreeqci_gui.h"
diff --git a/isotopes.cpp b/isotopes.cpp
index 21b8515c..fce9d49d 100644
--- a/isotopes.cpp
+++ b/isotopes.cpp
@@ -1325,25 +1325,19 @@ master_isotope_store(const char *name, int replace_if_found)
  */
 	int n;
 	struct master_isotope *master_isotope_ptr;
-	ENTRY item, *found_item;
-	char token[MAX_LENGTH];
 /*
  *   Search list
  */
-	strcpy(token, name);
-
-	item.key = token;
-	item.data = NULL;
-	found_item = hsearch_multi(master_isotope_hash_table, item, FIND);
-
-	if (found_item != NULL && replace_if_found == FALSE)
+	std::map<std::string, struct master_isotope*>::iterator mi_it =
+		master_isotope_map.find(name);
+	if (mi_it != master_isotope_map.end() && replace_if_found == FALSE)
 	{
-		master_isotope_ptr = (struct master_isotope *) (found_item->data);
+		master_isotope_ptr = mi_it->second;
 		return (master_isotope_ptr);
 	}
-	else if (found_item != NULL && replace_if_found == TRUE)
+	else if (mi_it != master_isotope_map.end() && replace_if_found == TRUE)
 	{
-		master_isotope_ptr = (struct master_isotope *) (found_item->data);
+		master_isotope_ptr = mi_it->second;
 		master_isotope_init(master_isotope_ptr);
 	}
 	else
@@ -1355,19 +1349,11 @@ master_isotope_store(const char *name, int replace_if_found)
 		master_isotope_ptr = master_isotope[n];
 	}
 	/* set name and z in pointer in master_isotope structure */
-	master_isotope_ptr->name = string_hsave(token);
+	master_isotope_ptr->name = string_hsave(name);
 /*
- *   Update hash table
+ *   Update map
  */
-	item.key = master_isotope_ptr->name;
-	item.data = (void *) master_isotope_ptr;
-	found_item = hsearch_multi(master_isotope_hash_table, item, ENTER);
-	if (found_item == NULL)
-	{
-		error_string = sformatf( "Hash table error in master_isotope_store.");
-		error_msg(error_string, CONTINUE);
-	}
-
+	master_isotope_map[name] = master_isotope_ptr;
 	return (master_isotope_ptr);
 }
 
@@ -1381,11 +1367,7 @@ master_isotope_alloc(void)
  *      return: pointer to a master_isotope structure
  */
 {
-	struct master_isotope *master_isotope_ptr;
-	master_isotope_ptr =
-		(struct master_isotope *) PHRQ_malloc(sizeof(struct master_isotope));
-	if (master_isotope_ptr == NULL)
-		malloc_error();
+	struct master_isotope *master_isotope_ptr = new struct master_isotope;
 /*
  *   set pointers in structure to NULL, variables to zero
  */
@@ -1427,7 +1409,7 @@ master_isotope_search(const char *name)
 /* ---------------------------------------------------------------------- */
 {
 /*
- *   Function locates the string "name" in the hash table for master_isotope.
+ *   Function locates the string "name" in the map for master_isotope.
  *
  *   Arguments:
  *      name    input, character string to be found in "master_isotope".
@@ -1436,21 +1418,15 @@ master_isotope_search(const char *name)
  *      pointer to master_isotope structure "master_isotope" where "name" can be found.
  *      or NULL if not found.
  */
-	struct master_isotope *master_isotope_ptr;
-	ENTRY item, *found_item;
-	char token[MAX_LENGTH];
+	struct master_isotope* master_isotope_ptr = NULL;
 /*
  *   Search list
  */
-	strcpy(token, name);
-
-	item.key = token;
-	item.data = NULL;
-	found_item = hsearch_multi(master_isotope_hash_table, item, FIND);
-
-	if (found_item != NULL)
+	std::map<std::string, struct master_isotope*>::iterator mi_it =
+		master_isotope_map.find(name);
+	if (mi_it != master_isotope_map.end())
 	{
-		master_isotope_ptr = (struct master_isotope *) (found_item->data);
+		master_isotope_ptr = mi_it->second;
 		return (master_isotope_ptr);
 	}
 	return (NULL);
diff --git a/mainsubs.cpp b/mainsubs.cpp
index fcebbacc..75ac03fb 100644
--- a/mainsubs.cpp
+++ b/mainsubs.cpp
@@ -61,10 +61,6 @@ initialize(void)
 	stag_data->exch_f = 0;
 	stag_data->th_m = 0;
 	stag_data->th_im = 0;
-/*
- *   Create hash tables
- */
-	hcreate_multi((unsigned) MAX_ELTS, &master_isotope_hash_table);
 
 	// user_print
 	user_print = (struct rate *) PHRQ_malloc((size_t) sizeof(struct rate));
diff --git a/structures.cpp b/structures.cpp
index 38303481..aaaefd80 100644
--- a/structures.cpp
+++ b/structures.cpp
@@ -192,11 +192,10 @@ clean_up(void)
 	/* master_isotope */
 	for (i = 0; i < (int)master_isotope.size(); i++)
 	{
-		master_isotope[i] = (struct master_isotope*)free_check_null(master_isotope[i]);
+		delete master_isotope[i];
 	}
 	master_isotope.clear();
-	hdestroy_multi(master_isotope_hash_table);
-	master_isotope_hash_table = NULL;
+	master_isotope_map.clear();
 	/* calculate_value */
 	for (i = 0; i < (int)calculate_value.size(); i++)
 	{