The dynamics of hydrated water molecules in Nafion membranes were studied by inspection of diffusion coefficients and relaxation times of ¹H nuclei using low-field nuclear magnetic resonance (NMR). Temperature-dependent measurements are an excellent tool to monitor the different mobility regimes. Bound states of water were analyzed using a dried membrane and the results obtained were used to determine the underlying processes of the mobile water molecules in a hydrated membrane. We observed a crossover between adsorption and diffusion processes as a function of temperature. At low temperatures, the low mobility of bound water enhances the adsorption of unbound water molecules, while at intermediate temperatures diffusion is the preponderant mechanism. At higher temperatures the morphological changes in the membranes affect the water-polymer interaction, favoring once more the adsorption processes.