A novel fluorescence microscopy technique for visualizing the transport of liquid water in unsaturated hydrophobic fibrous media has been developed and is applied to the gas diffusion layer of a PEM fuel cell. In the experiments, fluorescein dye solution is pumped through the fibrous hydrophobic gas diffusion layer (GDL) and imaged with fluorescence microscopy. Transient image intensity data is correlated to the liquid surface height and is analyzed and presented in the form of three-dimensional reconstructions of the time-evolving gas/liquid inter-face inside the fibrous structure. The high spatial resolution of the visualization can resolve the dynamic transport of liquid water through distinct pathways, which helps to refine understanding regarding liquid water transport mechanism within these porous layers. The physical observations suggest that the water is not transported via a converging capillary tree as suggested in prior work and models. Rather, transport is dominated by fingering and channeling. Based on the physical insight obtained from the experiments, a new water transport scheme is proposed as the basis for developing improved models for water transport in hydrophobic gas diffusion layers. (c) 2005 Elsevier B.V. All rights reserved.