The effect of the ionomer content on the mass transport ability in gas diffusion electrodes of proton exchange membrane fuel cells (PEMFCs) was investigated. The influence of the catalytic activity caused by a change in ionomer content was eliminated by adjusting the amount of the catalyst. The mass transport ability was evaluated by the current of the oxygen reduction reaction at 0.8-0.7 V versus RHE, after normalizing the catalytic activity of the electrodes using the current at 0.9 V as an index of the activity. From the results, the optimum content of the ionomer was obtained from the viewpoint of the compromise of the ionic access and the gas access. Furthermore, in order to clarify the extension of the carbon I ionomer interface, the electrical double layer capacitance was measured using electrodes without catalyst. Based on the results of these two series of experiments, the microscopic configuration of the ionomer impregnated inside the electrodes was discussed. (C) 2003 Elsevier Science B.V. All rights reserved.