The counter-ion transport-number of membrane affects columbic efficiency of electrochemical cells, many of which encounter varying salt concentration or temperature at different times of operation. Effect of NaCl concentration and temperature on the counter-ion transport number of commercial cation (Selemion CMV and Neosepta CMX) and anion (Selemion AMV and Neosepta AMX) exchange membranes was determined using chronopotentiometry. The transport number of the membranes didn't change with increasing NaCl concentration from 0.017 M to 0.1 M. Variation of transport number with temperature was determined from dronopotentiometric data using the Sand equation as well as a novel procedure. Values obtained from the Sand equation showed an anomalous trend, but the values obtained from the second procedure followed a consistent trend. The counter-ion transport number decreased significantly on increasing temperature by 20 degrees C and the decline was higher for the cation exchange membranes. (C) 2015 Elsevier B.V. All rights reserved.