Permeation experiments for Ar/CH4 and CO2/H-2 Mixtures across a SAPO-34 membrane were performed with the objective of highlighting their distinct diffusion characteristics. For the Ar/CH4 mixture, the effective transport coefficients of the individual components in the mixture were found to be practically the same as those obtained from unary permeation experiments, when compared at the same total loading at the upstream face of the membrane. There is no speeding-up of the tardier CH4. Only at the high loadings is there evidence of a small, about 10%, slowing-down of the more mobile Ar. A different picture emerges for permeation of CO2/H-2 mixtures across SAPO-34 membrane. While the transport coefficient of CO2 is practically identical to those obtained from unary experiments, there is a significant slowing-down of the more mobile H-2. At the highest loading, the transport coefficient of H-2 is about a third of the value of the pure component. Permeation of CO2/CH4 mixtures across an MFI membrane showed that there is a significant slowing-down of the more mobile CH4 With concomitant speeding-up of the tardier CO2. The incorporation of the Vignes interpolation formula for the exchange coefficient D-12 in the Maxwell-Stefan equations does not adequately reflect the slowing-down and speeding-up phenomena for either SAPO-34 or MFI membranes. (c) 2007 Elsevier B.V. All rights reserved.