Sulfonated poly(arylene ether sulfone) (S-PES) cation-exchange membranes with various degrees of the sulfonation were prepared via direct polymerization and characterized in terms of the electrochemical properties, including the surface heterogeneity and the current-voltage (I-V) relation. The S-PES membranes exhibited moderate swelling properties and ion-exchange capacities (IECs) for electro-membrane applications. Moreover, the membranes with the mole ratio of sulfonated monomers of over 40% showed excellent electrochemical properties. The electrical resistances were below 1.0 Omega cm(2) in a 0.5 mol dm(-3) NaCl and the ionic permselectivities were comparable to those of commercial membranes. The fraction of conducting regions, epsilon, on the membrane surface was evaluated through a chronopotentiometry. The epsilon values of the S-PES membranes were higher than those of the sulfonated poly(arylene ether sulfone) membranes prepared via conventional post-sulfonation route, leading to a low electrical resistance and a high ionic permselectivity. Considering the mechanical properties, the S-PES membrane with the mole ratio of sulfonated monomers of 40% was found to be very suitable for electro-membrane applications. (C) 2003 Elsevier Science B.V. All rights reserved.