Using an optical H 2 /air polymer electrolyte fuel cell ͑PEFC͒, the mechanics of liquid water transport, starting from droplet emergence on the gas diffusion layer ͑GDL͒ surface, droplet growth and departure, to the two-phase flow in gas channels, is characterized under automotive conditions of 0.82 A/cm 2 , 70°C, and 2 atm. It is observed that water droplets emerge from the GDL surface under oversaturation of water vapor in the gas phase, appear only at preferential locations, and can grow to a size comparable to the channel dimension under the influence of surface adhesion. Liquid film formation on more hydrophilic channel walls and channel clogging are also revealed and analyzed.Water management that balances membrane dehydration with electrode flooding is critical to achieve high performance and longevity of polymer electrolyte fuel cells ͑PEFCs͒. At high current density and/or low flow stoichiometry, PEFC is prone to flooding; that is, there is an excessive amount of water accumulated in the cell. If pores in the catalyst layer and gas diffusion layer ͑GDL͒ are filled with liquid water, or if the gas channels are clogged by liquid water to such an extent that the transport of reactant gases to the electrodes is hindered, substantially deteriorated cell performance results and mass transport limitation due to flooding occurs. The GDL, either nonwoven carbon paper or woven carbon cloth, is highly porous ͑Ͼ70% with pore sizes in the range of 10-30 m͒, electrically conductive, and hydrophobic. In addition, a microporous layer ͑MPL͒ ͑e.g., 30 m thick͒, consisting of carbon particles mixed with the PTFE binder, is usually applied onto the side of the GDL facing the catalyst layer. The MPL features a finer pore structure with a pore size on the order of 0.1-0.5 m. The MPL is intended to provide wicking of liquid water into the GDL by creating a gradient in liquid water pressure and minimize electric contact resistance with the adjacent catalyst layer. Wilson et al. 1 speculated that droplets of water generated at the interface of MPL and catalyst layer are in some form proportional in size to the diameter of MPL pores.Understanding liquid water transport and distribution in a PEFC is a key to unraveling the origin and development of flooding. Prior experimental efforts to probe the water distribution in an operating PEFC have included neutron radiography 2 and gas chromatography ͑GC͒ 3,4 measurements. The in situ method using neutron radiography was reported to investigate the two-phase flow pattern in the flowfield of both hydrogen and methanol PEFCs. Neutron beams can penetrate through a metal fuel cell to image the real-time liquid water profiles along the large-scale flowfield. However, the neutron radiographic imaging is currently limited in both spatial ͑e.g. Ͼ150 m͒ and temporal resolution ͑e.g. Ͻ30 Hz͒, making it difficult to capture two-phase flow phenomena in PEFC that is transient in nature and controlled by surface forces. Our previous work 3,4 on water distribution measurement by using a Micro GC provi...