The permeation of a series of gases with widely different Lennard-Jones kinetic diameters and sorption properties is investigated as a function of feed pressure (up to 100 kPa) and temperature (298-473 K) with two different methods. The membrane system studied consists of an MFI (silicalite) top-layer (thickness 3 μm) directly grown on an α-alumina support. Flux values of molecules with a kinetic diameter, d(k), < 0.45 nm (H2, CO2 and n-alkanes (CH4 to n-butane)) are in the range of 1.5-15 mmol m-2 s-1. Size exclusion is found for o-xylene (d(k) = 0.68 nm) and 2,2-dimethylbutane (d(k) = 0.60 nm). Flux values are obtained for iso-butane, SF6 and benzene, intermediate between those for small molecules and those for molecules with size exclusion. Maxima in the curve of flux versus temperature are found for gases with non-linear Langmuir-type adsorption curves as a function of pressure. The temperature of maxima at 100 kPa pressure increases in the order of CO2 < ethane < propane < butane and shifts to higher values with increasing pressure.