For the separation of binary mixtures several techniques can be considered, of which distillation is the most widely used. When problems occur in distillation (like the formation of an azeotropic mixture), membrane separation can be an alternative. A cascade of membranes has to be applied in those cases where it is impossible to achieve a complete separation in a single-membrane separation process. To calculate the separation in such a cascade, a McCabe-Thiele diagram is used in which the equilibrium curve is determined by the membrane selectivity. Optimization calculations for the cascade have been performed with respect to the total membrane surface area. Because of the large reflux flows around the feed stage, the permeate-retentate ratio in the feed stage has to be chosen carefully, since the required membrane surface area is determined mereiy by the permeate flows. The membrane characteristics that determine the required membrane surface area are selectivity and permeability: an increased selectivity or permeability reduces the membrane area. However, an increase of the selectivity usually goes with a decrease of the permeability. It appears from this study, that for selectivity and permeability values commonly found for reverse-osmosis (RO) membranes, an increase of the permeability affects the required membrane area to a larger extent than an increase in selectivity. This effect is illustrated by simulations for RO membranes used for the separation of water-l,3-butanedioi mixtures. The total membrane area can be reduced by 25% when two membranes are available with opposite selecttvtty, i.e. membranes with a retention for the other component.