This paper theoretically studies the water transport phenomena in PEM fuel cells, mainly investigating the transient behavior in the gas diffusion layer (GDL), catalyst layer (CL) and proton exchange membrane (PEM). In the PEM, both diffusion and electro-osmosis processes are considered, while in the GDL and CL, only diffusion process is taken into account. The process of water uptake is employed to account for the water transport at the interface between the PEM and CL. The results indicate that the water content in the PEM and the time for reaching the steady state in the start-up process are influenced by the humidification constant, k, the humidification, and the thickness of PEM. The rise of the k increases the water content in the membrane and shortens the time for reaching the steady state. Insufficient humidification causes relatively small water content and long steady time. When the PEM is thinner, the water is more uniformly distributed, the water content gets higher, and the time for reaching the steady state is distinctly shorter. (c) 2006 Elsevier B.V. All rights reserved.