A simple dynamic model was proposed to describe the transient output characteristics of polymer electrolyte membrane fuel cells (PEMFCs). Physical parameters for the proposed model were estimated based on the steady and transient results experimentally obtained using a commercial PEMFC (1.2 kW Nexa power module, Ballard Power Systems Inc.). In the transient experiments, the load current was varied according to the high-low current step and the high-low current sweep schedules. The practical applicability of the proposed dynamic model was demonstrated by comparing the measured and the predicted output behaviors of the Nexa PEMFC power module for prescribed load current schedules. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.