Using a micro X-ray beam, the structure of a water swollen Nafion((R)) membrane, alone or in a membrane electrode assembly (MEA) designed for fuel cells, was studied upon cooling down to -70 degrees C. By scanning the membranes along their thicknesses, the water sorption-desorption process was investigated as a function of cooling/heating stages. From the scattering curves, it was deduced that the state of the water at a sub-zero temperature is glassy inside the membrane and ice crystals are observed only outside it. In the case of the MEA, this growth can be destructive since this formation is localised inside the active layers. (c) 2007 Published by Elsevier B.V.