In this work, we investigate transport and reaction kinetics in agglomerates of cathode catalyst layers used in proton-exchange membrane fuel cells. Two types of spherical agglomerates are evaluated, which represent limiting structures that can be obtained by distinct synthetic procedures. One type consists of a mixture of carbon/catalyst particles and a proton conducting perfluorosulfonated ionomer (PFSI). The other type consists of carbon/catalyst particles and water-filled pores. The performance of the former type is rationalized on the basis of the well-known Thiele-modulus. Characteristics of the latter type are studied using Nernst-Planck and Poisson equations. Aspects of current conversion, reactant and current distributions and catalyst utilization are explored. In general, the PFSI-filled agglomerates exhibit more homogeneous distributions of reaction rates. Effectiveness factors for these are close to one. However, it was found that proton penetration depths in water-flooded agglomerates could be quite substantial under certain conditions, resulting in unexpectedly high catalyst utilization. The effects of agglomerate radius and of boundary conditions on the agglomerate surface are studied. An approximate analytical solution was obtained for a planar geometry of agglomerates. The significance of these results for the optimization catalyst layers in the light of operation conditions and synthesis methods is discussed. (C) 2004 Elsevier B.V. All rights reserved.