It is known that the mass transfer characteristics of gas diffusion layers (GDLs) are closely related to cell performance in PEFCs. In this study, a method for measuring oxygen diffusivity in microporous media by means of an oxygen sensor based on a galvanic cell has been developed. This method is expected to be useful for measuring oxygen diffusivity in microporous media in a wet condition. Oxygen diffusivity in two kinds of microporous GDL media was evaluated. Experimental results indicate that diffusivity in microporous media cannot be determined on the basis of porosity alone, but that it may be influenced by some other factor, such as tortuosity. Additionally, there are some GDL materials that exhibit anisotropic diffusivity. (C) 2009 Elsevier Ltd. All rights reserved.