Zeolitic materials are often used for the separation of hydrocarbon mixtures. In this paper we suggest a procedure for modelling the separation performance using a combination of two techniques: 1. Configurational-bias Monte Carlo (CBMC) simulations for estimating the required pure component and mixture isotherms, and 2. Maxwell-Stefan formulation of mixture diffusion in zeolites. The applicability of the suggested approach is demonstrated by comparing the theoretical predictions with experimental data published on zeolite membrane permeation. The theory provides clues to the development of new separation techniques and also for optimisation of the operating conditions.