Modeling two-phase flow in proton exchange membrane (PEM) fuel cells is hampered by a lack of conceptual understanding of flow patterns in the gas diffusion layer (GDL). In this paper, pore-scale visualizations of water in different types of GDLs were used to improve Current understanding of flow and transport phenomena in PEM fuel cells. Confocal microscopy was used to capture the real-time transport of water, and pressure micro-transclucers were installed to measure water breakthrough pressures. Three types of fuel cell GDLs were examined: TO series (Toray Corp., Tokyo,Japan), SGL series (SGL Carbon Group. Wiesbaden, Germany), and MRC series (Mitsubishi Rayon Corp., Otake City, Japan). The Visualizations and pressure measurements revealed that despite difference in "pore" structures in the three types of GDLs, waterfollowed distinct flow paths spanning several pores with characteristics similar to the "column flow" phenomena observed previously in hydrophobic or coarse-grained hydrophilic soils. The results obtained from this study can aid in the construction of theories and models for optimizing water management in fuel cells. (C) 2009 Elsevier B.V. All rights reserved.X