The dynamic performance of a PEM fuel cell is one of the most important criteria in the design of fuel cells, especially when the application of the fuel cell in mobile systems is concerned. To attack this issue, we extend the theoretical model developed by Okada (J. Electroanal. Chem. 465 (1999) 1,18) to an unsteady state model and investigate the transient behavior of water transport across the membrane as well as the influences of several physical parameters on the characteristic time to reach the steady state. We also consider the influence due to the presence of foreign impurity ions, which turn Out to be a crucial factor affecting the unsteady state features of water transport across the membrane. The results suggest that a higher initial current density to start the operation, a higher water flux from the anode side, it smaller operational current density, and a lower level of contamination in the membrane (especially at the cathode side) can all result in a shorter time for the water transport to reach the steady state, and thus a better dynamic performance of the fuel cell can be obtained. (C) 2003 Elsevier B.V. All rights reserved.