The performance of a gas diffusion layer comprised of a macro porous and micro porous layer has been studied both experimentally and by numerical simulation. Experimental data at different humidification conditions have been compared to full cell, three-dimensional computational fluid dynamics calculations to validate the physical model of the cell. Local distributions of current density, electrochemical variables, temperature, and gas composition are discussed in detail. Model calculations agree well with experimental data and the solutions with and without the micro porous layer show that this layer has an effect on the overall performance and the local distributions show differences. The effect of hydrogen dilution is also explored in this paper with micro/macro gas diffusion media. The results reveal that proton exchange membrane fuel cell (PEMFC) performance depends not only on the oxygen but also on the hydrogen partial pressure. (c) 2006 Elsevier B.V. All rights reserved.