Water uptakes properties and effective fixed charge concentration have been determined in aqueous electrolyte solutions (LiCl, NaCl and KCl) for different commercial sulfonated polymer cation-exchange membranes with different morphology. Differences in the water uptake properties and in the membrane effective fixed charge concentration has been found, which have been analyzed on the basis of the different membrane structures. The experimental results show correlations between the water uptake and the loss of the membrane selectivity at high electrolyte concentrations which are dependent on the membrane morphology. Relationships are found which permit to estimate the membrane effective fixed charge concentration from equilibrium and morphological properties with the advantage of avoiding the need for membrane potential measurements. (C) 2011 Elsevier Ltd. All rights reserved.