A model is developed which describes the mass transfer in ion-selective membranes as used in the chloralkali electrolysis process. The mass transfer model is based on the Maxwell-Stefan theory, in which the membrane charged groups are considered as one of the components in the aqueous mixture. The Maxwell-Stefan equations are re-written in such a way that the current density can be used as an input parameter in the model, which circumvents an extensive numerical iterative process in the numerical solution of the equations. Because the Maxwell-Stefan theory is in fact a force balance, and the clamping force needed to keep the membrane charged groups in its place is not taken into account, the model is basically over-dimensioned: the mole fraction of the membrane can be calculated by using the equivalent weight (EW) of the membrane or by using the equations of continuity. In this work, the latter method has been chosen. The results of the computer model were verified in several ways, which show that the computer model gives reliable results. Several exploratory simulations have been carried out for a sulfonic layer membrane and the conditions as encountered in. the chloralkali electrolysis process. As there are no (reliable) Maxwell-Stefan diffusivities available for a Nafion membrane, in this trend study the diffusivities were all chosen equal at a more or less arbitrary value of 1.10(-10) m(2) s(-1). Due to this, the absolute values of several performance parameters are incorrect as compared with industrial chloralkali operation (e.g. an unrealistically high current efficiency of 95.7% was found), but the model can still be used to obtain trends. For example, it is shown that the thickness of the membrane hardly increases the current efficiency (CE), however, the required potential drop proportionally increases with thickness; The pH rapidly increases to values greater than 12 just inside the membrane at the anolyte side. Moreover, for different values of the pH in the anolyte, the pH profiles inside the membrane nearly coincide with each other. A change in the anolyte strength does not have a significant effect on the performance of the membrane. At low values of the current density, a high value of the current efficiency is found. However, this is not due to a low OH- counter flux, but to the simultaneous transport of OH- and Cl- towards the catholyte.