Water permeabilities at 70 degrees C through Nafion membranes, ranging in thickness from 6 to 201 mu m are reported. Three types of water permeation are described: hydraulic permeation (liquid-liquid permeation), pervaporation-like permeation (liquid-vapour permeation) and vapour permeation (vapour-vapour permeation). Permeabilities for liquid-liquid permeation increase with decreasing membrane thickness. Permeabilities for liquid-vapour permeation and vapour-vapour permeation, increase with decreasing membrane thickness but are independent of thickness for <56 mu m thick membranes. Internal and interfacial water transport resistances for Nafion are estimated. The contribution of the interfacial transport resistance is greater than half the total transport resistance for 201 mu m thick membranes and increases further with decreasing membrane thickness - explaining why vapour-vapour permeation and liquid-vapour permeation fluxes do not increase with further decreases in membrane thickness below 56 mu m. In situ net water fluxes through operating MEAs are measured at 70 degrees C. Operating conditions that require significant back permeation of water were chosen: "dry-anode/wet-cathode" and "dry" conditions. In both conditions, liquid-vapour permeation is largely responsible for the back permeation at high current density regime (>0.6 A cm(-2)). Small pressure difference across the assembled ultra-thin membranes (6 mu m) was found to enhance the rate of back permeation at high current density operation (>0.6 A cm(-2)). (C) 2010 Elsevier B.V. All rights reserved.