Water sorption, desorption, and permeation in and through Nafion membranes 51-606 pm thick were measured as functions of temperature between 30 and 90°C. Water permeation rates are weakly dependent on membrane thickness. Water permeation from liquid water was faster than water permeation from 100% relative humidity gas. These results indicated that interfacial mass transport at the membrane/gas interface was the limiting resistance to transport and not diffusion. Water sorption is initially limited by interfacial mass transport; above a critical water content water sorption is limited by polymer relaxation dynamics associated with swelling. Water desorption from a water saturated film was an order of magnitude faster than water sorption. Different regimes of water transport and sorption in Nafion membranes corresponding to diffusion, interfacial transport and polymer swelling depend on membrane thickness, temperature and the contacting phase.