Visualization of the oxygen partial pressures was carried out at the surface of a gas diffusion layer (GDL) for the first time together with the upper part of the gas-flow channel of the cathode of a running polymer electrolyte fuel cell (PEFC) using two different oxygen-sensitive luminescent dye films. The visualized distributions of the oxygen partial pressures at the GDL and the upper gas-flow channel during the PEFC operation were very different in a conventional test cell. The change in the distribution of the oxygen partial pressures was observed by changing the oxygen utilization, which should be connected with the reactive locations in the membrane-electrode assembly (MEA). A numerical calculation was carried out to understand the distributions of water and current density inside the MEA. The water distribution inside the MEA was confirmed to strongly affect the distributions of the current density and the oxygen /partial pressure. (C) 2014 Elsevier B.V. All rights reserved.