diff --git a/PBasic.cpp b/PBasic.cpp
index 2c56b1a1..b0016d0d 100644
--- a/PBasic.cpp
+++ b/PBasic.cpp
@@ -1617,6 +1617,9 @@ listtokens(FILE * f, tokenrec * l_buf)
 		case tokdiff_c:
 			output_msg("DIFF_C");
 			break;
+		case toksetdiff_c:
+			output_msg("SETDIFF_C");
+			break;
 		case toksa_declercq:
 			output_msg("SA_DECLERCQ");
 			break;
@@ -3823,7 +3826,25 @@ factor(struct LOC_exec * LINK)
  			n.UU.val = (parse_all) ? 1 : PhreeqcPtr->diff_c(str);
 		}
 		break;
+	
+	case toksetdiff_c:
+		{
+			double d;
+
+			require(toklp, LINK);
+
+			const char * str = stringfactor(STR1, LINK);
+			require(tokcomma, LINK);
+
+			// double arugument
+			d = realexpr(LINK);
+			require(tokrp, LINK);
+
+ 			n.UU.val = (parse_all) ? 1 : PhreeqcPtr->setdiff_c(str, d);
 			
+			//PhreeqcPtr->PHRQ_free((void *) str);
+		}
+		break;
 	case tokval:
 		l_s = strfactor(LINK);
 		tok1 = LINK->t;
@@ -7159,7 +7180,8 @@ const std::map<const std::string, PBasic::BASIC_TOKEN>::value_type temp_tokens[]
 	std::map<const std::string, PBasic::BASIC_TOKEN>::value_type("viscos_0",           PBasic::tokviscos_0),
 	std::map<const std::string, PBasic::BASIC_TOKEN>::value_type("rho_0",              PBasic::tokrho_0),
 	std::map<const std::string, PBasic::BASIC_TOKEN>::value_type("current_a",          PBasic::tokcurrent_a),
-	std::map<const std::string, PBasic::BASIC_TOKEN>::value_type("pot_v",              PBasic::tokpot_v)
+	std::map<const std::string, PBasic::BASIC_TOKEN>::value_type("pot_v",              PBasic::tokpot_v),
+	std::map<const std::string, PBasic::BASIC_TOKEN>::value_type("setdiff_c",          PBasic::toksetdiff_c)
 };
 std::map<const std::string, PBasic::BASIC_TOKEN> PBasic::command_tokens(temp_tokens, temp_tokens + sizeof temp_tokens / sizeof temp_tokens[0]);
 
diff --git a/PBasic.h b/PBasic.h
index f2f39cc9..a943186d 100644
--- a/PBasic.h
+++ b/PBasic.h
@@ -323,6 +323,7 @@ public:
 		tokequiv_frac,
 		tokcallback,
 		tokdiff_c,
+		toksetdiff_c,
 		toksa_declercq,
 		tokedl_species,
 		tokviscos,
diff --git a/Phreeqc.h b/Phreeqc.h
index 16f2f4a6..f91a0178 100644
--- a/Phreeqc.h
+++ b/Phreeqc.h
@@ -101,6 +101,7 @@ public:
 	LDBLE log_activity_coefficient(const char *species_name);
 	LDBLE aqueous_vm(const char *species_name);
 	LDBLE diff_c(const char *species_name);
+	LDBLE setdiff_c(const char *species_name, double d);
 	LDBLE sa_declercq(double type, double sa, double d, double m, double m0, double gfw);
 	LDBLE calc_SC(void);
 	/* VP: Density Start */
diff --git a/basicsubs.cpp b/basicsubs.cpp
index 4d803dfd..b4aa5f63 100644
--- a/basicsubs.cpp
+++ b/basicsubs.cpp
@@ -158,7 +158,7 @@ diff_c(const char *species_name)
 	LDBLE g;
 
 	s_ptr = s_search(species_name);
-	if (s_ptr != NULL && s_ptr->in != FALSE && s_ptr->type < EMINUS)
+	if (s_ptr != NULL /*&& s_ptr->in != FALSE && s_ptr->type < EMINUS*/)
 	{
 		g = s_ptr->dw;
 		if (s_ptr->dw_t)
@@ -173,6 +173,29 @@ diff_c(const char *species_name)
 }
 /* ---------------------------------------------------------------------- */
 LDBLE Phreeqc::
+setdiff_c(const char *species_name, double d)
+/* ---------------------------------------------------------------------- */
+{
+	struct species *s_ptr;
+	LDBLE g;
+
+	s_ptr = s_search(species_name);
+	if (s_ptr != NULL)
+	{
+		s_ptr->dw = d;
+		g = s_ptr->dw;
+		if (s_ptr->dw_t)
+				g *= exp(s_ptr->dw_t / tk_x - s_ptr->dw_t / 298.15);
+		g *= viscos_0_25 / viscos;
+	}
+	else
+	{
+		g = 0;
+	}
+	return (g);
+}
+/* ---------------------------------------------------------------------- */
+LDBLE Phreeqc::
 calc_SC(void)
 /* ---------------------------------------------------------------------- */
 {