Transient characteristics of proton exchange membrane (PEM) fuel cells with parallel and interdigitated flow fields upon changes in voltage load were investigated by applying a three-dimensional, two-phase model. Effects of channel to rib width ratios and cathode inlet flow rates on the transient response of PEM fuel cell were examined in detail. Current overshoot and undershoot occur because the time scale for the voltage change is much shorter than for the oxygen concentration changes. Therefore, the oxygen concentrations on the cathode diffusion layer-catalyst layer interface immediately after the voltage changes are essentially the same as before the voltage changes, which results in higher reaction rates causing overshoots when the voltage decreases or lower reaction rates causing undershoots when the voltage increases. The predictions also show that as the voltage decrease rate is reduced, the overshoot peak weakens and the response time shortens. Since the interdigitated flow field has higher oxygen concentrations on the cathode diffusion layer-catalyst layer interface due to the forced convection, the overshoot peaks and the undershoot valleys are all greater than for the parallel flow field. For both flow fields, larger channel to rib width ratios cause larger overshoots, smaller undershoots and longer response times. (C) 2011 Elsevier Ltd. All rights reserved.