An increasing number of hybrid processes consisting of conventional unit operations and vapour permeation modules have been suggested in recent years. To facilitate accurate process design, rigorous models of this novel membrane process are essential. By comparing experimental results obtained from a pilot plant, it was shown that simply solving the material balances and assuming constant permeances is not sufficient to predict the operating behaviour of a vapour permeation module. In order to reflect the experimental results it was necessary to consider the concentration, temperature and pressure dependency of the permeation process through the membrane as well as the mass transfer restriction in the boundary layer on the feed side. The introduced model is capable of taking account of these effects and predicts the experimental results satisfactorily. The model was written using the commercial, equation-oriented process simulator Aspen Custom Modeler(TM) and hence can easily be integrated with commercial process simulation packages. (C) 2003 Society of Chemical Industry.