Gas diffusion through porous media is critical for the high current density operation of a polymer electrolyte membrane fuel cell, where the electrochemical reaction becomes rate limited by the diffusive flux of reactants. Precise knowledge of the diffusivity through various components in a fuel cell is necessary for accurate modeling and analysis. However, many experimental measurements of diffusivity in literature have high measurement uncertainty. In this study, an improvement to the accuracy of the Loschmidt cell method is presented for measuring the diffusivity through materials with a submillimeter thickness. The diffusivity through various gas diffusion layers (GDLs) is measured, and the relative differences between GDLs are explained using scanning electron microscopy and the method of standard porosimetry. The experimental results from this study and others in current literature are used to develop a generalized correlation for the diffusibility as a function of porosity in the through-plane direction of GDLs. (C) 2012 American Institute of Chemical Engineers AIChE J, 59: 1409-1419, 2013