Suppression of methanol crossover in polymer electrolyte membranes is a key technology for direct methanol fuel cells (DMFCs). We have prepared a Nafion(R) 117 wherein platinum nanocrystals are highly dispersed and subsequently used it in a DMFC. This is expected to suppress the short-circuit reaction by oxidizing methanol at the Pt catalysts. Indeed, this is observed by the appreciable increase in the cathode potential, resulting from the reduced amount of methanol reaching the cathode.