The results of a fundamental mass transport model based on the Maxwell-Stefan approach are compared to experimental data obtained by Akzo-Nobel for a Dupont Nafion ion-selective membrane as used in chloralkali electrolysis processes. The main problem in the application of the Maxwell Stefan based mass transfer model to the chloralkali electrolysis process is a lack of available diffusivities for the membrane. Estimation of these diffusivities in the membrane based on a method presented by Wesselingh et al. (1995. Chem. Engng J., 57, 75-89) gave unrealistic high membrane potential drops. Therefore, another method was followed. First, a sensitivity analysis was carried out which resulted in a reduced set consisting of the dominating Maxwell-Stefan diffusivities. First estimates of these remaining diffusivities were determined for single layer sulfonic and a carboxylic membranes. With a slight adjustment of the values of the diffusivities obtained for the separate sulfonic and carboxylic layers, the performance parameters of the DuPont Nafion membrane could be predicted well for a reference experiment. These diffusivities also proved to be suitable for other anolyte strengths. However, for other catholyte strengths and current densities these diffusivities (even after a correction for the water uptake according to the method of Wesselingh et al. (1995. Chem. Engng. 5., 57, 75-89)) did not result in a good agreement between the simulated and experimentally observed performance parameters. Only after a correction of the diffusivities the simulations yielded approximately the same performance parameters as experimentally observed. From this it can be concluded that although a fundamental model is used in order to describe the mass transfer in a membrane, a single set of diffusivities is not sufficient in order to obtain the experimentally observed performance parameters at different process conditions. At this moment there is not enough knowledge on the exact phenomena taking place in the membrane in order to predict the necessary corrections of the diffusivities a priori. As long as there are no theoretically founded and reliable relations available to predict the Maxwell-Stefan diffusivities in a membrane (or accurate experimental data for these diffusivities) only a semi-empirical method as used in this study can serve as a basis for a further progress in the development of an existing (in this case DuPont Nafion) membrane.