A microscopic catalyst model is proposed for predicting the local mass diffusion of a single catalyst particle in the cathode catalyst layer. The numerical model is employed to obtain a geometric description of the active layer. The cathode catalyst particle model proposed here is based on the microstructure of the catalyst layer. The catalyst particle is treated as many small platinum particles (1-10.0 nm) embedded in the larger carbon particle support (30-100 nm). This assembly is surrounded by an ionomer film with thickness ranging from 0.5 nm to 10.0 nm. The modeling results confirm that the platinum particle size, platinum loading and ionomer thickness can each play an important role on local mass and charge transport in the PEM fuel cell catalyst particle agglomerate. The local spherical diffusion, reactant distribution and electrochemical kinetics are strongly influenced by particle size, platinum loading and ionomer thickness. (C) 2008 Elsevier Ltd. All rights reserved.