Isotopic transient permeation of methanol/ethanol, inethanol/2-propanol, and methanol/acetone mixtures through a MFI zeolite membrane was investigated experimentally. For the methanol/2-propanol and methanol /ethanol mixtures, the more mobile species (methanol) was slowed down in the mixture and the tardy species (2-propanol or ethanol) was speeded up. The extent of slowing down and speeding up depended on the mixture composition. The Maxwell-Stefan (M-S) diffusion equations reproduced the observed mixture permeation results, at least qualitatively, provided the self-exchange coefficient D-ii for each species was taken to be a tenth of the pure component M-S diffusivity D-i. For the methanol/acetone mixtures, both species slowed clown in the mixture. Adjusting the value of the self-exchange coefficient Dii in the M-S equations did not provide an explanation of the observed experimental results; it appeared that the component diffusivities, D-1 and D-2, in the mixture were both lower than the values of the pure components, an effect that has not earlier been reported in the literature. (c) 2007 Elsevier B.V. All rights reserved.