An in-depth insight in the role of gas diffusion layers (GDLs) and its impact on the water management is a key issue for the optimization of fuel cells. A new ex situ test method is developed to investigate the water transport in gas diffusion media for polymer electrolyte membrane fuel cells (PEMFCs). This research is focused on properties of GDLs, which influence the water removal and water retention in the cell. Gas diffusion media are evaluated ex situ in terms of liquid water and water vapor transport employing a conventional PEMFC setup. The amount of water transported through a GDL and Out of the cell is determined by the properties of the gas diffusion medium. GDL properties such as the GDL thickness have a significant impact on water transport behavior. Furthermore, high impregnation weight or an additional micro-porous layer (MPL) reduces water removal due to enhanced mass transport resistances. The composition and distribution of the impregnation material in the GDL substrate also play a crucial role. Water transport rates depend not only on the GDL properties but increase exponentially with cell temperature. Finally, a two-phase water transport model is proposed taking into account both diffusive gas phase and liquid water transport in diffusion media. Based on this model, ex situ data set in correlation with in situ performance in PEMFCs on dry operating conditions and guidelines towards new design concepts for gas diffusion media are deduced. (C) 2008 Elsevier B.V. All rights reserved.