The permeation of water through Nafion membranes has been measured as a function of film, temperature and water activities. Water permeation coefficients increased with water content in Nafion. For water activity in the range 0 - 0.8, permeation coefficients increased linearly with water activity and scaled inversely with membrane thickness. The permeation coefficient was independent of membrane thickness when the membrane was in contact with liquid water (aw=1). Water transport is shown to be consistent with water diffusion through the hydrophilic phase of Nafion. The volume fraction of the hydrophilic phase increases with water activity, which correlates with the increase in the permeation coefficient. As aw approaches 1, the hydrophilic domain volume increases sufficiently whereby the limiting resistance to water transport changes from diffusion to interfacial transport across the membrane/gas interface. The mechanical and transport properties are linked to structural changes in the microphases in Nafion induced by water absorption.