The slow dynamic behavior of mass transfer in pervaporation has been reported by different authors. Until now, however, only a little experimental data and no model capable to describe this phenomenon have been available. A new detailed model for mass transfer in pervaporation, based on the dual-sorption theory, Maxwell-Stefan diffusion, and the UNIQUAC model, is proposed. This model is capable of properly describing mass transfer at steady state and during transients. Model parameters for two different PVA/PAN membranes for ethanol/water separation are given. The model shows good agreement with both steady state and dynamic experimental data.