Two component permeation measurements have been performed by the Wicke-Kallenbach method on a thin (3 μm) zeolite MFI (Silicalite-1) membrane with molecules of different kinetic diameters, d(k). The membrane was supported by a flat porous α-Al2O3 substrate. The results obtained could be classified in six regimes based on differences in: Occupation degrees, θ, on the external surface. Occupation degrees, θ, in the zeolite pores. Mobilities in the zeolite pores. High separation factors are obtained for mixtures of 'weakly/strongly adsorbing' gases and for mixtures of 'small/large' gases. Separation by size-exclusion is demonstrated for 8 kPa n-C6H14/16 kPa 2,2-dimethylbutane (α > 600, T = 298-473 K) and 0.31 kPa p-xylene/0.26 kPa o-xylene (α > 200, T = 400 K). At low θ (high temperature, low pressures) the permeation of both components in a mixture equals the single-gas permeation of these components. At high a (low temperature and high pressure) the permeation of both components in mixtures is different from single-gas permeation values. The single-gas permeance of 'small' molecules with kinetic diameter d(k) < 0.45 nm ranges from 1 x 10-8 to 15 x 10-8 mol m-2 s-1 Pa-1 in the temperature range 298-473 K. The permeation decreases with increasing d(k) of the molecules.