The behaviour of water molecules confined in the perfluorinated ionomer membrane, which is widely used for polymer electrolyte fuel cells, was investigated by H-1 NMR spectroscopy. The H-1 NMR spectra of water molecules confined in the membrane presented characteristic features depending on temperature and hydrated conditions. On the basis of the observations of the variations in the H-1 NMR signals. it was deduced that the polymer underwent very slow conversions among distinct morphologies depending on temperatures and humidified conditions below 348 K. A plausible scheme of the local conversion of the states of the water molecules associated with the morphological changes of the polymer was proposed. (C) 2008 Elsevier B.V. All rights reserved.