Sulfonated polystyrene-type membranes were synthesized by plasma polymerization of a mixture of styrene and trifluoromethane sulfonic acid monomers in a low-frequency after-glow discharge plasma reactor. Such a deposition process enables the preservation of the monomers structure, which was confirmed by mass spectrometry analysis. The synthesized plasma-polymerized membranes are dense and uniform with a few microns thickness. Their structure determined by Fourier-transform infra-red spectroscopy and X-ray photoelectron spectroscopy is very rich in sulfonic acid groups (up to 5%) and stable up to 120 degrees C. Even if their intrinsic proton conductivity is low (10(-1) mS cm(-1)), directly related to their disorganized and highly cross-linked structure, plasma-polymerized membranes present a proton conduction ability similar to Nafion (R) because of their low thickness. Due to their highly cross-linked structure, these membranes enable a reduction of the methanol crossover in a factor 10 by comparison with Nafion (R). Thus, the integration of plasma-polymerized films in miniaturized direct methanol fuel cells as proton-exchange membranes seems promising. (c) 2005 Elsevier B.V. All rights reserved.