The mass transfer characteristics of the gas diffusion layer (GDL) are closely related to the performance of polymer electrolyte fuel cells. This study investigates the configuration of a new GDL in which two porous media with different wettabilities are alternately arranged (hybrid GDL). The oxygen diffusivity characteristics with respect to water content (saturation) were measured using an experimental system that employs a galvanic oxygen sensor as an oxygen absorber. Furthermore, X-ray radiography was used to observe the internal water distribution in microporous media for GDL to elucidate the enhancement mechanisms of oxygen diffusivity of the new microporous media. It was possible to distinguish between voids and water in microporous media by using X-ray computer tomography. In addition, the water distributions in the new microporous media were visualized and the mechanisms for the high oxygen diffusivity of the hybrid structure of microporous media were clarified. Copyright (c) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.