High-resolution neutron radiography was used to image an operating proton exchange membrane fuel cell in situ. The cross-sectional liquid water profile of the cell was quantified as a function of cell temperature, current density, and anode and cathode gas feed flow rates. Detailed information was obtained on the cross-sectional water content in the membrane electrode assembly and the gas flow channels. At low current densities, liquid water tended to remain on the cathode side of the cell. Significant liquid water in the anode gas flow channel was observed when the heat and water production of the cell were moderate, where both water diffusion from the cathode and thermal gradients play a significant role in determining the water balance of the cell. Within the membrane electrode assembly itself, the cathode side was moderately more hydrated than the anode side of the assembly from 0.1 to 1.25 A cm(-2). The total liquid water content of the membrane electrode assembly was fairly stable between current densities of 0.25 and 1.25 A cm(-2), even though the water in the gas flow channels changed drastically over this current density range. At 60 degrees C, the water content in the center of the gas diffusion layer was depleted compared to the membrane or gas flow channel interfaces. This phenomenon was not observed at 80 degrees C where evaporative water removal is prevalent. (c) 2008 The Electrochemical Society.