To maintain the optimum performance of polymer electrolyte fuel cells by keeping the high ion conductivity of polymer electrolyte membranes, appropriate water managements of the membranes are required. H-1 NMR signals of water confined in the perfluorinated ionomer membranes used for PEFCs presented characteristic change after heating. At around 358 K, the resonance line reduced substantially, and the spectral feature was retained for a considerably long time ( more than 18 days) after the temperature reverted to 297 K. The result suggests that the polymer structure was converted at around 358 K, so that the mobility of water molecules became higher and their chemical environments became more homogeneous, and that the structure was stable at room temperature. The structure is considered to be stabilized most likely by the interaction of water molecules to hold the structure even at 297 K, which keeps the mobility and homogeneity of water molecules high.