Polycarbonate and polysulfone based heterogeneous cation-exchange membranes were prepared and their electro-chemical characterization was carried out by membrane potential and chronopotentiometry studies in sodium chloride solution of 0.01 M concentration. Apparent and true counter-ion transport numbers were estimated by membrane potential and chronopotentiometric measurements, respectively, for both types of membranes with cation-exchange resin content ranging in between 30% and 40% (w/w). The difference in permselectivity values, estimated by both techniques for the same membrane under comparable experimental conditions, was explained assuming that the heterogeneous ion-exchange membrane consisted of conducting and non-conducting phases and/or water transport through the membrane along with counter-ion. Using a modified Sand's equation, the fraction of conducting phase in the membrane material was estimated. (C) 2004 Elsevier B.V. All rights reserved.