The separation of binary mixtures of hexane isomers using ZSM-5 zeolite membranes on porous stainless steel and alumina tubes was modeled for both vapor permeation and pervaporation. The fluxes and selectivities for single-component and mixture permeation of n-hexane, 3-methylpentane (3-MP), and 2,2-dimethylbutane (DMB) were described by combining two diffusion mechanisms. The flux through zeolite pores was modeled by Maxwell-Stefan diffusion without molecular interaction, whereas the flux through the grain boundaries between the crystals was modeled by activated Knudsen diffusion. Adsorption isotherms and diffusion coefficients for the zeolite pores were obtained from the literature. Two fitting parameters for the size and the amount of non-zeolite pores were included in the model to determine membrane quality. The influence of membrane support, membrane preparation, operation mode, and process parameters on the separation characteristics is illustrated with the transport model. (C) 2003 Elsevier B.V. All rights reserved.