In order to suppress methanol crossover in direct methanol fuel cells (DMFCs), a new polymer electrolyte membrane (PEM) has been developed. Platinum nanocrystals were highly dispersed in a Nafion(R) 117 film (Pt-PEM) to catalyze the oxidation of the crossover methanol with O-2. An appreciable increase in the cathode potential was observed, resulting from the reduced amount of methanol reaching the cathode. Distribution profiles of the specific resistance (rho) in thickness direction were measured by using Pt probes inserted into the Pt-PEM under various operation conditions. Humidification of oxygen gas was found to be effective to achieve both uniform rho distribution and high cathode performance. At a DMFC, Pt-Ru anode uPt-PEMu Pt cathode, operated with 1 M CH3OH and humidified O-2 at 90degreesC, the terminal voltage at 200 mA/cm(2) was 0.41 V (IR-free) or 0.36 V (IR included).