Influences of the concentration of different electrolyte solutions on the water content of the CMS cation exchange membrane were studied. Based on the microheterogeneous model of electromembrane, we determined the fraction of the interstitial phase in presence of NaCl and H2SO4 solutions by using the conductivity results obtained from impedance spectroscopy. The polyelectrolyte theory was applied to explain the dependence of the water content and the fraction of the interstitial phase on the nature of the counter ions. (c) 2006 Elsevier B.V. All rights reserved.