Polycrystalline randomly oriented defect free zeolite layers on porous alpha-Al2O3 supports are prepared with a thickness of less than 5 mu m by in situ crystallisation of silicalite-1. The flux of alkanes is a function of the sorption and intracrystalline diffusion. In mixtures of strongly and weakly adsorbing gases and at high loadings of the strongly adsorbing molecule in the zeolite pore, the flux of the weakly adsorbing molecule is suppressed by the sorption and the mobility of the strongly adsorbing molecule resulting in pore-blocking effects. The separation of these mixtures is mainly based on the sorption and completely different from the permselectivity. At low loadings of the strongly adsorbing molecules the separation is based on the sorption and the diffusion and is the same as the permselectivity. Separation factors for the isomers of butane (n-butane/isobutane) and hexane (hexane/2,2-dimethylbutane) are respectively high (10) and very high (> 2000) at 200 degrees C. These high separation factors are a strong evidence that the membrane shows selectivity by size-exclusion and that transport in pores larger than the zeolite MFI pores (possible defects, etc) can be neglected.