The hydration profile of proton-exchange membranes (PEM) in running fuel cells (FC) are obtained through an original method that employs very-high-energy synchrotron-radiation X-ray diffraction in space/time-resolved measurements. Determining the amount and spatial distribution of water in the polymeric membrane of PEMFCs under working conditions is of utmost importance in PEMFC technology, since FC performances are strongly dependent on PEM hydration.