The fuel crossover and internal current in a polymer electrolyte membrane fuel cell undergo a chemical reaction in the cell without power generation. These are the main phenomena for reduced cell voltage at low current density. This fuel crossover also degrades the fuel cell performance, efficiency, and durability. Thus, observation of these phenomena is important for understanding and developing a polymer electrolyte membrane fuel cell. Using X-ray radiography, the water distribution and membrane swelling, which indicate fuel crossover and internal current, in an operating polymer electrolyte membrane fuel cell under open-circuit conditions were examined. The X-ray images effectively demonstrated the transient changes of each phenomenon, which are related to the properties of each component and the operating conditions. (C) 2011 Elsevier B.V. All rights reserved.