The water transport in a proton exchange membrane fuel cell (PEMFC) is investigated in this study. A five-layer theoretical model is proposed that includes anode and cathode gas diffusion layers (GDLs), catalyst layers (CLs), and the layer of proton exchange membrane. Especially, the volume of membrane is assumed to be variable with its water content and this effect on water transport is examined. Both steady and transient transport phenomena are considered by changing several crucial system parameters such as the relative humidity of reactant gas, the porosity of GDL, and the membrane thickness. The results show that if the humidification of the reactant gases is sufficient, the water management would be better for larger porosities of GDLs or a thinner membrane, and the resistance and overvoltage of the membrane can be reduced significantly as well. Furthermore, it is found that the membrane swelling effect will increase the water content of the membrane especially in the region close to the cathode interface, and lengthen the response time for a PEMFC to reach steady state as switching between two different operating conditions in comparison with the case ignoring this effect. (c) 2006 Elsevier B.V. All rights reserved.