A model is developed to describe sorption or desorption of water from planar Nafion membranes with one face contacting stagnant water or saturated water vapor and the opposite face exposed to flowing gas. Interfacial evaporation/condensation kinetics is considered along with bulk water transport. Simplifying assumptions yield analytical formulas that relate the steady-state or transient system response to the gas flow rate. The approximate predictions correlate well with experimental data gathered under various conditions. Water transport rates are found to be limited by interfacial vaporization-exchange kinetics, rather than bulk percolation or diffusion, when membrane slabs are thinner than approximately 300 mu m. (C) 2008 Elsevier B.V. All rights reserved.