Pore-filling electrolyte membranes have been prepared for use as electrolyte membranes in a direct methanol fuel cell. The pores of a porous substrate were filled with a polymer electrolyte, with the membrane swelling being suppressed by the substrate matrix. Proton conductivity occurred through the filling electrolyte polymer. Swelling of the electrolyte polymer was used to control methanol permeation, and the substrate had good mechanical strength at high temperature. We developed a membrane that consisted of a poly(vinylsulfonic acid/acrylic acid) crosslinked gel in a porous polytetrafluoroethylene (PTFE) substrate. This had a high proton conductivity with reduced membrane methanol permeability, and was thermally stable to 130 degreesC.