A spherical flooded-agglomerate model for the cathode catalyst layer of a proton exchange membrane fuel cell, which includes the kinetics of oxygen reduction, at the catalyst I electrolyte interface, proton transport through the polymer electrolyte network, the oxygen diffusion through gas pore, and the dissolved oxygen diffusion through electrolyte, is considered. Analytical and numerical solutions are obtained in various control regimes. These are the limits of (i) oxygen diffusion control, (ii) proton conductivity control, and (iii) mixture control. The structure and material parameters, such as porosity, agglomerate size, catalyst layer thickness and proton conductivity, on the performance are investigated under these limits. The model could help to characterize the system properties and operation modes, and to optimize catalyst layer design. Crown Copyright (C) 2004 Published by Elsevier Ltd. All rights reserved.