Highly efficient proton-exchange membranes (PEMs) have been made by using polymer blends of water soluble poly(vinyl alcohol) and poly(styrene sulfonic acid-co-maleic acid) (i.e. PVA/PSSA-MA). Upon successive thermal and chemical treatments of the membranes. the methanol permeability decreased more than one order of magnitude compared with Nafion 117 but the proton conductivity was as high as 0.1 S cm(-1) at room temperature. The degrees of swelling of the membranes were controlled by adding a sui table crosslinker (i.e. glutaraldehyde) and varying the mole ratios of maleic acid groups to sulfonic acid groups. The characteristic hydrophilic/hydrophobic separation length of the crosslinked membrane (equilibrated in water) was shown to be smaller than that of Nafion 117 as inferred from the results of small-angle X-ray scattering (SAXS) analysis. Moreover, we could not observe the typical ionomer peak in the PVA/PSSA-MA membrane equilibrated with methanol from the SAXS profile, demonstrating very poor methanol affinity of the membrane matrix. It is believed that this structural feature contributes to the reduction of methanol permeability through the membranes. (C) 2004 Elsevier B.V. All rights reserved.