In polymer electrolyte membrane fuel cells (PEMFC), Nation is used not only as a membrane placed between the anode and the cathode, but also as an important component of the active layer in a gas diffusion electrode. The content of Nation ionomer in the electrode affects simultaneously the gas permeability, the catalytic activity and the ionic resistance. Hence, an optimum Nation content in the catalyst layer is necessary for good performance. This study examines the dependence of optimum Nafion content in the catalyst layer on platinum loading. Investigations are also made of the variation in cell impedance, electrochemical surface area (ESA) and percentage Pt utilization with change in Nafion loading in the catalyst layer. It has been generally reported that the optimum Nation loading is in the range of 30-36 wt.%, irrespective of the platinum loading. The present study indicates that the optimum Nafion loading should infact depend on the platinum loading. For electrodes with different platinum loadings of 0.5, 0.25 and 0.1 Mg pt cm(-2), the best performance was obtained at different Nafion loadings of 20, 40 and 50 wt.%, respectively. It is also found that the optimum Nafion loading does not depend on the membrane thickness. (C) 2004 Elsevier B.V. All rights reserved.