New polymer electrolyte membranes for polymer-electrolyte-membrane (PEM) fuel cells (PEFCs) have been developed. Platinum nanocrystals (d = 1-2 nm) were highly dispersed in a Nafion 112 film (Pt-PEM, thickness 50 mu m) to catalyze the recombination of the crossover H-2 with O-2, and the water generated was found to humidify the Pt-PEM directly. In order to clarify the self-humidifying properties in the Pt-PEM, the amount of water vapor produced by the recombination and the faradaic reaction was analyzed together with those of consumed H-2 and O-2 by monitoring humidity in the exhausting gases from PEFCs operated with dry H-2 and O-2 at 80 degrees C. The H-2 used for the self-humidification increased with increasing output current density, but the total H-2 consumption decreased to ca. 2/3 of that in normal membranes. All H2O produced inside the Pt-PEM was found to be exhausted from the anode, resulting in the efficient humidification of the membrane on the anode side, which is dried by electro-osmotic drag. Thus, the resistance of the Pt-PEM was lowered to 0.04 Omega cm(2). It is also found that the Pt-PEM improved the cathode potential distinctively which was ascribed to elimination of the chemical reaction of crossover gases in the cathode catalyst layer, and eliminated any disturbance of O-2 diffusion by H2O vapor produced by the reaction. The operation of PEFCs with minimal or no humidification by using Pt-PEMs is essential in applications to power sources for electric vehicles or various electronic devices from the viewpoints of the simplification of control systems, cold starts : or response to abrupt load changes.