One factor influencing the performances of polymer electrolyte membrane fuel cells (PEMFCs) is the degree of hydration of their membranes. Until now, only theoretical calculations of the number of water molecules contained in the membranes, few studies of the distribution of water in working fuel cell membranes and ex situ measurements, have been carried out. In the present study, we report the first experimental determination of a dehydration process occurring in the polymeric membrane of a fuel cell, observed in real time by in situ X-ray diffraction (XRD) of a very high-energy synchrotron radiation. (C) 2004 The Electrochemical Society.