A single polymer electrolyte fuel cell (PEFC), normally operated at 60-80 degrees C, was operated at 90 degrees C. The in-plane temperature distribution on the back of separator at the cathode was measured by thermograph for various combination of polymer electrolyte membrane (PEM) and gas diffusion layer (GDL) thickness and relative humidity of supply gases, and the difference in heat and mass transfer characteristics and power generation characteristics under different conditions were investigated. With the thickest PEM, Nafion 115, the influence of the relative humidity of supply gas on the power generation characteristics decreased with decreasing GDL thickness. With a thin GDL, PEM is humidified by the enhanced transfer of water from the high to the low relative humidity side. By the same effect, in-plane temperature distribution from the inlet to the outlet was flat. On the other hand, investigation of the impact of thickness of PEM on the power generation characteristics with a thin GDL, TGP-H-030, the relative humidity of supply gas had little effect on the power generation characteristics regardless of PEM thickness, because PEM and catalyst layer are humidified by the enhanced water transfer through the thin GDL. However, the power generation performances of Nafion NRE-212 and Nafion NRE-211 were higher than that of the thicker Nafion 115, since the thinner PEMs were thought to have lower ohmic resistance. The in-plane temperature distribution with Nafion 115 and Nafion NRE-211 was flat, while that with Nafion NRE-212 increased from the inlet to the outlet as a result of the power generation performance and water transfer characteristics. The combination of the thinnest PEM and GDL, Nafion NRE-211 and TGP-H-030, was found to be the optimal for the high temperature operation at 90 degrees C from the viewpoint of the power generation performance as well as control of the in-plane temperature distribution.