In situ confocal micro-Raman spectroscopy was used to probe the water distribution in an electrolyte membrane in a polymer electrolyte fuel cell (PEFC) under various cell-operating conditions. The water content, lambda(R) (number of water molecules per sulfonic acid group), in a Nafion (R) membrane was calculated from the intensity of the O-H stretching (nu(O-H)) band. By analyzing the Raman spectra as a function of the membrane depth in an operating PEFC, the lambda(R) distributions in the membrane were obtained under different temperature, humidification, current density and gas-flow rate conditions. The nu(O-H) intensity in the electrolyte membrane increased with increasing current density, relative humidity, and gas utilization. The water distribution in the electrolyte membrane can be understood as a balance among back-diffusing water produced from the cathode, electro-osmotic drag, and water removal via the gas diffusion layer (GDL), all of which is information that will be important for the realization of cell operation without humidification in future advanced fuel cell vehicles. (C) 2012 Published by Elsevier Ltd.