We measured the self-diffusion coefficients of water in a Nafion membrane and two sulfonated polyethersulfone (SPES) membranes with varying ion-exchange capacities (IEC) in terms of relative humidity using the pulse field gradient NMR (PFG-NMR) technique. The selfdiffusion coefficients were plotted against the number of water molecules per sulfonic acid group, 1, and compare these values with the results of molecular dynamics (MD) simulations. Classical MD simulations for all membranes were carried out using a consistent force field at 1=3, 6, 9, 12, and 15. The dynamic properties of water (H2O) and hydronium (H3O+) on a molecular level were estimated as self-diffusion coefficients and residence times around a sulfonate group (SO3-). The diffusion coefficients of H2O and H3O+ followed the order, Nafion>SPES with IEC=1.4 >SPES with IEC=1.0>SPES with IEC=0.75, which agreed with the experimental data. The residence time distribution of H2O around SO3- in Nafion was in the range of 1–6 ps, whereas H2O in the SPES exhibited a residence time of greater than 20 ps.