diff --git a/opm/polymer/PolymerProperties.cpp b/opm/polymer/PolymerProperties.cpp
index 4234a55b7..af7edb349 100644
--- a/opm/polymer/PolymerProperties.cpp
+++ b/opm/polymer/PolymerProperties.cpp
@@ -212,7 +212,8 @@ namespace Opm
         eff_relperm_wat = relperm[0]/rk;
         if (if_with_der) {
             deff_relperm_wat_ds = (drelperm_ds[0]-drelperm_ds[2])/rk; //derivative with respect to sw
-            deff_relperm_wat_dc = dc_ads_dc*relperm[0]/(rk*rk*c_max_ads_);
+            //\frac{\partial k_{rw_eff}}{\parital c} = -\frac{krw}{rk^2}\frac{(RRF-1)}{c^a_{max}}\frac{\partial c^a}{\partial c}.
+            deff_relperm_wat_dc = -(res_factor - 1)*dc_ads_dc*relperm[0]/(rk*rk*c_max_ads_); 
         } else {
             deff_relperm_wat_ds = -1.0;
             deff_relperm_wat_dc = -1.0;