The rates of oxygen reduction and generation of water vapor in a gas diffusion electrode and an active catalyst layer of a proton exchange membrane fuel cell were analytically studied. In particular, the analytical model accounts for the diffusion of oxygen and water vapor, the reaction of oxygen and hydrogen ion in the liquid and the effectiveness factor of a porous Pt catalyst on the cathode side. Calculations of cell polarization behavior are compared with existing experimental data. There is a great possibility that the rate limiting process is transport of oxygen and hydrogen ion in the Pt catalyst particle when the effective diameter of the Pt catalyst particle is greater than 2 mum. (C) 2001 Elsevier Science Ltd. All rights reserved.