Molecular dynamics simulations were carried out to determine the self-diffusivities of CH4 and CO2 both for pure components and in 50-50 mixtures for a range of molar loadings in MFI, CHA and DDR zeolites. In CHA and DDR zeolites, that consist of cages separated by narrow windows, the inter-cage hopping of molecules occur practically independent of one another and, consequently, the diffusivities of pure components are the same as in the mixture. In sharp contrast, in MFI that consists of intersecting channels, the more mobile species is slowed down significantly in the mixture. (c) 2006 Elsevier B.V. All rights reserved.