Two component permeation measurements have been performed by the Wicke-Kallenbach method on a thin (3 mu m) zeolite MFI (Silicalite-1) membrane with molecules of different kinetic diameters, d(k). The membrane was supported by a flat porous alpha-Al2O3 substrate. The results obtained could be classified in six regimes based on differences in:Occupation degrees, theta, on the external surface.Occupation degrees, theta 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 (alpha>600, T=298-473 K) and 0.31 kPa p-xylene/0.26 kPa o-xylene (alpha>200, T=400 K). At low a thigh 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 dk <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 dk of the molecules.