Proton-conducting membranes for a direct methanol fuel cell, based on styrene-(ethylene-butylene)-styrene (SEBS) triblock copolymer blended with polyvinylidene fluoride (PVDF), have been developed. First, the partially sulfonated SEBS with a variety of degrees of substitution was prepared by reacting the SEBS with propionyl sulfate. Then, the sulfonated SEBS was blended with PVDF at various blending ratios and fabricated by using a solution casting technique. The water uptake, proton conductivities, methanol permeabilities, and mechanical properties of the blend membranes were measured by using gravimetry, impedance analysis, gas chromatography, and tensile test, respectively. It was found that water uptake of the blend membranes increased with the sulfonated SEBS content, at the expense of their methanol resistance. The proton conductivity of the blend membranes, however, did not change linearly with the sulfonated SEBS content. This was related to poor compatibility between the two polymers in the blend membranes. However, by adding 5 wt % of poly(styrene)-b-poly(methyl methacrylate) block copolymer, compatibility, proton conductivity, and methanol resistance of the blend membrane increased. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 393-399, 2010