This paper compares three different approaches for describing the cathode catalyst layer of a PEMFC, using a three-dimensional CID model. The three catalyst treatments include: a thin-film model, a discrete-catalyst volume model and an agglomerate model. It is shown that, within a single-phase approach using physically meaningful parameters, the commonly employed thin-film or discrete-volume descriptions of the catalyst layer do not show the significant mass transport limitations which occur at higher current densities; while this region is observed using a catalyst agglomerate approach. Further, an in-depth analysis of the current density distributions indicates that for a given electrode overpotential the thin film model significantly over-predicts the current density, compared to the discrete and agglomerate approaches. The thin-film model also greatly exaggerates the variation in current density both along and across the channel. Finally, the agglomerate model predicts noticeable mass transport losses even at very low current densities despite the use of high stoichiometric ratios and thin-electrolyte films. (C) 2008 Elsevier B.V. All rights reserved.