A small amount of basic polymer was incorporated in the Nafion membrane. Compared with the re-cast Nafion membrane, the Nafion/basic polymer membrane reduced the methanol permeability considerably. The equilibrium water uptake and proton conductivity decreased, but the thermal and mechanical stability was enhanced with increasing concentration of basic polymer. These property changes were caused by formation of cation/anion complex between acidic Nafion and basic polymer molecules. The effects of the types and molecular weights of basic polymers on the methanol permeability and proton conductivity were not significant.