Synchrotron X-ray tomography is used to visualize the water distribution in gas diffusion layers (GDL) and flow field channels of a polymer electrolyte membrane fuel cell (PEMFC) subsequent to operation. An experimental setup with a high spatial resolution of down to 10 mu m is applied to investigate fundamental aspects of liquid water formations in the GDL substrate as well as the formation of water agglomerates in the flow field channels. Detailed analyses of water distribution regarding the GDL depth profile and the dependence of current density on the water amount in the GDL substrate are addressed. Visualizations of water droplets and wetting layer formations in the flow field channels are shown. The three-dimensional insight by means of this quasi in situ tomography allows for a better understanding of PEMFC water management at steady state operation conditions. The effect of membrane swelling as function of current density is pointed out. Results can serve as an essential input to create and verify flow field simulation outputs and single-phase models. (C) 2010 Elsevier B.V. All rights reserved.