The dynamic response of PEM (Proton exchange membrane) fuel cells is a complex phenomenon which is affected by numerous factors related to their designs and operating conditions. Despite that experimental data is available in the literature, a systematic numerical study to explain the dynamic behavior of PEM fuel cells is currently unavailable. In this paper, a one-dimensional, two-phase, dynamic model of PEM fuel cell is developed to achieve this principal objective. Transient profiles of cell voltage, activation and ohmic over-potentials, saturation level of liquid water, oxygen concentration, and membrane water content are predicted under various operating conditions. Under constant fuel and air flow rates, it is found that the cell voltage exhibits undershoot behavior following a step increase in current density due to the inherent time delay experienced by the redistribution of membrane water content with a response time of similar to 50s. The undershoot is followed by an overshoot in the presence of flooding with a significantly longer predicted response time of similar to 150-200 s. It is found that the various operating conditions mainly affect the specific details of the undershoot and overshoot profiles without changing their general behavioral forms. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.