The electrochemical characteristics of lab-made cation exchange membranes based on a PS/DVB-ETFE film with different degrees of grafting (37% and 44%, respectively) have been evaluated in presence of sulfuric acid by means of current-voltage, chronopotentiometric and conductivity measurements. The transport number of the hydrogen ion in the membranes is estimated by resorting to the Sand equation. The chronopotentiometric curves of the membranes in contact with 0.1 M H2SO4 solution present the typical shape of overall homogeneous membranes. The validity of the two-phase model (interstitial solution and gel phase) has therefore been tested with the aim of contributing to provide a better description of the membrane microheterogeneity. According to the two-phase model, the volume fraction of the interstitial phase in the membranes is determined from the membrane and solution conductivity data. Furthermore, the relationship between the interstitial phase volume fractions and the water content of the membranes is discussed on the basis of the polyelectrolyte theory. (C) 2009 Elsevier Ltd. All rights reserved.