Water absorption, membrane swelling, and self-diffusivity of water in 1100 equivalent weight Nafion were measured as functions of temperature and water activity. Free volume per water at 80 degrees C, determined from water uptake and volume expansion data, decreases with water content in the membrane from 12 cm(3)/mol at lambda = 0.5 H2O/SO3 to 1.5 cm(3)/mol at lambda = 4. The change in free volume with water content displays a transition at lambda = 4. Limiting water self-diffusivity in Nafion was determined by pulsed gradient spin echo NMR at long delay times. The limiting self-diffusivity increases exponentially with water activity; the rate of increase of diffusivity with water content shows a transition at lambda = 4. The tortuosity of the hydrophilic domains in Nafion decreased from 20 at low membrane water activity to 3 at lambda = 4. It suggested a change in the connectivity of the hydrophilic domains absorbed water occurs at lambda similar to 4. The diffusivity results were employed to separate the contributions of diffusional and interfacial resistance for water transport across Nafion membranes, which enabled the determination of the interfacial mass transport coefficients. A diffusion model was developed which incorporated activity-dependent diffusivity, volume expansion, and the interfacial resistance, and was used to resolve the water activity profiles in the membrane.