diff --git a/Cantera/src/thermo/MargulesVPSSTP.cpp b/Cantera/src/thermo/MargulesVPSSTP.cpp
index bc2a11b7f..f2923c44d 100644
--- a/Cantera/src/thermo/MargulesVPSSTP.cpp
+++ b/Cantera/src/thermo/MargulesVPSSTP.cpp
@@ -916,25 +916,25 @@ namespace Cantera {
 	g0 = (m_HE_b_ij[i] - T * m_SE_b_ij[i]) / RT;
 	g1 = (m_HE_c_ij[i] - T * m_SE_c_ij[i]) / RT;
 	
-	//	dlnActCoeffdlnN_diag_[iK] += 2*(delBK-XB)*(g0*(delAK-XA)+g1*(2*(delAK-XA)*XB+XA*(delBK-XB)));
+	dlnActCoeffdlnN_diag_[iK] += 2*(delBK-XB)*(g0*(delAK-XA)+g1*(2*(delAK-XA)*XB+XA*(delBK-XB)));
 
-	double gfac = g0 + g1 * XB;
-	double gggg = (delBK - XB) * g1;
+// 	double gfac = g0 + g1 * XB;
+// 	double gggg = (delBK - XB) * g1;
 	
 
-	dlnActCoeffdlnN_diag_[iK] += gfac * delAK * ( - XB + delBK);
+// 	dlnActCoeffdlnN_diag_[iK] += gfac * delAK * ( - XB + delBK);
 	
-	dlnActCoeffdlnN_diag_[iK] += gfac * delBK * ( - XA + delAK);
+// 	dlnActCoeffdlnN_diag_[iK] += gfac * delBK * ( - XA + delAK);
 	
-	dlnActCoeffdlnN_diag_[iK] += gfac * (2.0 * XA * XB - delAK * XB - XA * delBK);
+// 	dlnActCoeffdlnN_diag_[iK] += gfac * (2.0 * XA * XB - delAK * XB - XA * delBK);
 	
-	dlnActCoeffdlnN_diag_[iK] += (delAK * XB + XA * delBK - XA * XB) * g1 * (-XB + delBK);
+// 	dlnActCoeffdlnN_diag_[iK] += (delAK * XB + XA * delBK - XA * XB) * g1 * (-XB + delBK);
 	
-	dlnActCoeffdlnN_diag_[iK] += gggg * ( - 2.0 * XA * XB + delAK * XB + XA * delBK);
+// 	dlnActCoeffdlnN_diag_[iK] += gggg * ( - 2.0 * XA * XB + delAK * XB + XA * delBK);
 	
-	dlnActCoeffdlnN_diag_[iK] += - g1 * XA * XB * (- XB + delBK);
+// 	dlnActCoeffdlnN_diag_[iK] += - g1 * XA * XB * (- XB + delBK);
       }
-      //   dlnActCoeffdlnN_diag_[iK] = XK*dlnActCoeffdlnN_diag_[iK]-XK;
+      dlnActCoeffdlnN_diag_[iK] = XK*dlnActCoeffdlnN_diag_[iK];//-XK;
     }
   }
 
@@ -949,7 +949,7 @@ namespace Cantera {
   void MargulesVPSSTP::s_update_dlnActCoeff_dlnN() const {
     int iA, iB;
     doublereal delAK, delBK;
-    double XA, XB, g0 , g1;
+    double XA, XB, g0 , g1, XK,XM;
     double T = temperature();
     double RT = GasConstant*T;
  
@@ -961,8 +961,9 @@ namespace Cantera {
      *  Loop over the activity coefficient gamma_k
      */
     for (int iK = 0; iK < m_kk; iK++) {
+      XK = moleFractions_[iK];
       for (int iM = 0; iM < m_kk; iM++) {
-
+	XM = moleFractions_[iM];
 	for (int i = 0; i <  numBinaryInteractions_; i++) {
     
 	  iA =  m_pSpecies_A_ij[i];    
@@ -983,22 +984,26 @@ namespace Cantera {
 	  g0 = (m_HE_b_ij[i] - T * m_SE_b_ij[i]) / RT;
 	  g1 = (m_HE_c_ij[i] - T * m_SE_c_ij[i]) / RT;
 
-	  double gfac = g0 + g1 * XB;
-	  double gggg = (delBK - XB) * g1;
+	  dlnActCoeffdlnN_(iK,iM) += g0*((delAM-XA)*(delBK-XB)+(delAK-XA)*(delBM-XB));
+	  dlnActCoeffdlnN_(iK,iM) += 2*g1*((delAM-XA)*(delBK-XB)*XB+(delAK-XA)*(delBM-XB)*XB+(delBM-XB)*(delBK-XB)*XA);
+
+// 	  double gfac = g0 + g1 * XB;
+// 	  double gggg = (delBK - XB) * g1;
 
 
-	  dlnActCoeffdlnN_(iK, iM) += gfac * delAK * ( - XB + delBM);
+// 	  dlnActCoeffdlnN_(iK, iM) += gfac * delAK * ( - XB + delBM);
 
-	  dlnActCoeffdlnN_(iK, iM) += gfac * delBK * ( - XA + delAM);
+// 	  dlnActCoeffdlnN_(iK, iM) += gfac * delBK * ( - XA + delAM);
 	  
-	  dlnActCoeffdlnN_(iK, iM) += gfac * (2.0 * XA * XB - delAM * XB - XA * delBM);
+// 	  dlnActCoeffdlnN_(iK, iM) += gfac * (2.0 * XA * XB - delAM * XB - XA * delBM);
 	
-	  dlnActCoeffdlnN_(iK, iM) += (delAK * XB + XA * delBK - XA * XB) * g1 * (-XB + delBM);
+// 	  dlnActCoeffdlnN_(iK, iM) += (delAK * XB + XA * delBK - XA * XB) * g1 * (-XB + delBM);
 
-	  dlnActCoeffdlnN_(iK, iM) += gggg * ( - 2.0 * XA * XB + delAM * XB + XA * delBM);
+// 	  dlnActCoeffdlnN_(iK, iM) += gggg * ( - 2.0 * XA * XB + delAM * XB + XA * delBM);
 
-	  dlnActCoeffdlnN_(iK, iM) += - g1 * XA * XB * (- XB + delBM);
+// 	  dlnActCoeffdlnN_(iK, iM) += - g1 * XA * XB * (- XB + delBM);
 	}
+	dlnActCoeffdlnN_(iK,iM) = XM*dlnActCoeffdlnN_(iK,iM);
       }
     }
   }