The dynamic performance of PEM fuel cells (PEMFCS) during startup or load change processes is an important factor in PEMFC design and operation, especially for automotive applications where dynamic response is crucial. In this study, a two-dimensional, isothermal, transient model has been developed. For comparison purposes, both steady-state and transient analysis have been conducted. The variation of reactants concentration, activation overpotential, reaction rate and corresponding current density distribution in the catalyst layer (CL) are analyzed in detail. The steady-state results indicate that the current density distribution is closely related to the activation overpotential in the CL. Meanwhile, this model is capable of predicting various transient phenomena as the cell experiences load changes in working conditions. The transient results demonstrate that the dynamic behavior of fully humidified PEMFC is mainly determined by the cathode flooding conditions and oxygen transport in the gas diffusion layer; the transient variations in the dry cathode case are well within 0.3 s, while this variation periods are prolonged to around I s if the cathode is severely flooded. Hence, it can be concluded that the dynamic responses of a cell with partially flooded cathode is in the range of 0.3-1 s time period. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.