A new type of proton conducting membrane was fabricated by multi-layer acid-base complex formation on a porous PE-g-PSS film. Polystyrene was initially grafted on a porous PE substrate using a supercritical carbon dioxide (SC-CO2) medium, and the grafted polystyrene was subsequently sulfonated to yield the PE-g-PSS membrane. The multi-layer acid base complex structure was obtained by the repetitive alternating deposition of poly(vinylimidazole) (PVIm) and poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA) on the PE-g-PSS film. The multi-layer acid-base complex structure was designed to suppress methanol crossover due to the more compact pore-filling structure as well as to control the hydrophilicity. As expected, the methanol permeability of the optimized multi-layer membrane was much lower, however, the proton conductivity of the membrane was higher than that of Nafion 115. (c) 2006 Elsevier B.V. All rights reserved.