MFI membranes with Si/Al ratios between 57 and ∞ were prepared on metallic and ceramic supports as part of ongoing work to develop membranes for membrane reactor applications. The membranes were characterized by H 2 and SF 6 single gas permeation measurements under various conditions, by mixture permeation studies of H 2 :SF 6 =1:1, and of n-pentane:i-pentane=1:1 as well as by permporosimetry in order to estimate the relative contributions of inter- and intracrystalline membrane transport. The permeation properties are mainly influenced by the Al content in the MFI layers but not by the type of the support. From permporosimetry and the n/i-pentane and H 2 /SF 6 mixture permeation it follows that the intercrystalline permeance increases with increasing Al content and therefore the mixture selectivities are low for Al-rich MFI membranes. The ZSM-5 membrane with the highest Al content (Si/Al=57) behaves exceptionally since it separates H 2 /SF 6 mixtures but not n/i-pentane mixtures. The permeation results depend strongly on the measurement conditions such as the pressure difference across the membrane and are influenced by the calcination parameters of the membrane.