H-2 T-1 NMR relaxation was used to characterize the molecular motion of deuterated water ((H2O)-H-2) in Aquivion E87-05, Nafion 117, and sulfonated-Radel proton-exchange membranes. The presence of bound water with solid character was confirmed by the dependence of the H-2 T-1 relaxation on the magnetic field of the spectrometer. By comparing the H-2 T-1 relaxation times of the different membranes that were equilibrated in varying humidities, the factors that influence the state of water in the membranes were identified. At low levels of hydration, the molecular motion of (H2O)-H-2 is affected by the acidity and mobility of the sulfonic acid groups to which the water molecules are coordinated. At higher levels of hydration, the molecular motion of (H2O)-H-2 is affected by the phase separation of the hydrophilic/hydrophobic domains and the size of the hydrophilic domains.