The component fluxes of binary mixtures through MFI-type membranes are strongly affected by the adsorption behaviour of the permeating species. An equilibrium adsorption study with the novel TEOM (tapered element oscillating microbalance) technique has shown a quantitative difference in the pure component isotherms of linear and branched C-6 alkanes in silicalite-1 crystals, wherein the branched isomers show a two-step adsorption behaviour. The concentration-dependent diffusivity is significantly affected by the dual-site nature of the adsorption. Configurational-entropy effects tend to favor the adsorption of linear over branched alkanes in isomer mixtures in silicalite-1 because linear molecules "pack" more efficiently in the structure. A permeation model, based on single-component parameters and accounting for these configurational-entropy effects, is used to predict the fluxes of binary mixtures across a silicalite-1 membrane. Good agreement with the available experimental data indicates the strength of the proposed model approach. (C) 2003 Elsevier Science B.V. All rights reserved.