A simple mathematical model for water diffusion in Nafion membranes was derived assuming that membrane properties are independent of membrane water content (water diffusion coefficient and membrane thickness), an equilibrium exists at the interface between channel and membrane, and the relationship between membrane water content and channel water concentration is linear. General analytical expressions were obtained as well as approximations. Comparison with three published experimental data sets, obtained with Nafion membranes separating a gas stream and a liquid water stream, showed good agreement, with an error generally smaller than 20%. (c) 2006 The Electrochemical Society.