The biocompatibility of contact lenses is closely related to their oxygen permeability. In hydrogel lenses, this characteristic can be attributed to the water permeability resulting from a combination of viscous and diffusive fluxes. Hydrogel lenses were studied by means of nuclear magnetic resonance (NMR) relaxation times, resulting in a mathematical model which evaluated the water self-diffusion coefficient as a quantification of the diffusive contribution to permeation. Comparing the results obtained with the data of permeability to oxygen as measured by other techniques, a reasonable agreement was shown for lenses with a higher water content (WC) with respect to lenses with a lower WC: this difference was accounted for by considering the different contribution to permeation.