Numerical simulations are performed to investigate the effect on PEM fuel cell performance of Nation loading in the catalyst layer. The investigation also considers variations of geometric parameters. A model that accounts for the volume fractions of Nation, the solid catalyst particles, and the void space inside the catalyst layers is incorporated into a three-dimensional computational fluid dynamics code, capable of resolving three-dimensional mass, momentum, and species transport phenomena as well as the electron- and proton-transfer processes in a PEMFC. Numerical results are first compared with experiments, showing close agreement between predictions and measurements. A parametric study on the effects of Nation loading and geometric parameters variation is carried out to evaluate the performance of PEMFC for various parameter combinations. (c) 2006 Elsevier B.V. All rights reserved.