Using an optical H-2/air polymer electrolyte fuel cell (PEFC), the mechanics of liquid water transport, starting from droplet emergence on the gas diffusion layer (GDL) 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 degrees 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. (C) 2004 The Electrochemical Society.