Self-humidification behaviors of polymer electrolyte membranes (PEMs) were clarified experimentally in polymer electrolyte fuel cells (PEFCs) operated with dry H-2 and O-2. New PEMs (Pt-PEM, TiO2-PEM, Pt-TiO2-PEM) were prepared by dispersing small amounts of Pt (1-2 nm in diameter) and/or metal-oxides such as TiO2 (5 nm in diameter) in Nafion 112 or a recasted Nafion film (normal-PEM, ca. 50 mu m in thickness). Distribution profiles of the specific resistances (rho) in thickness direction were measured with monitoring Pt-probes inserted into the PEMs, In normal-PEM, the rho increases with increase of current density at the anode side and vice versa at the cathode side, and this was pronounced at high current density. However, it was found that the rho decreased monotonically at every portion in Pt-PEM with increase of current density, although the rho of anode side is larger than that of cathode side. Further uniform rho distribution was achieved in Pt-TiO2-PEM, i.e., less than or equal to 20 Omega cm at a practically operational current density. The mechanisms of such a distinctive self-humidification and a suppression of crossover of reactant gases in the new PEMs were clarified by monitoring consumed H-2 and O-2 and produced water in exhausting gases from PEFCs in comparison with normal-PEM. In Pt-TiO2-PEMs, crossover H-2 and O-2 were recombined on Pt particles and all of the water generated inside the PEMs was exhausted from the anode. The TiO2 particles enhanced the back-diffusion of water produced by faradic reaction at the cathode by the hygroscopic property, resulting in very efficient humidification of the PEM of the anode side dried by the electroosmotic drag, It was also found that the new PEMs improve the cathode potential distinctively, which was ascribed to elimination of the short-circuit reaction of crossover gases in the cathode catalyst layer, resulting in a small non-faradic consumption of H-2 and no disturbance of reactant O-2 diffusion by the produced water vapor.