Structural characteristics and membrane performance of polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (sSEBS) copolymer proton exchange membranes in water-swollen and different ratio of water/methanol-swollen are studied by small angle X-ray scattering (SAXS), ATR FT-IR, AFM. SAXS profile of sSEBS membrane showed that microstructure was a hexagonally cylindrical structure and it was maintained in the existence of methanol in water until 30 wt.%. As the main interest was on the development of proton exchange membranes for fuel cells operating at lower temperature with liquid methanol feed, a selective thin layer was introduced on the top of sSEBS membrane by simple plasma treatment in the presence of maleic anhydride in order to prevent methanol crossover. Both methanol permeability and proton conductivity gradually decreased with increasing loading amounts of maleic anhydride. Hydrophobic anhydride properties on the top of sSEBS membrane act as a barrier for methanol but also for proton, resulting in decreasing methanol permeability and proton conductivity. The hydrolysis of anhydride groups to carboxylic acid provides a facilitated transport site for the proton. After hydrolysis, the proton conductivity was recovered and the recovery rate of proton conductivity by hydrolysis was higher than that of methanol permeability.