The Maxwell-Stefan (M-S) formulation is shown to be the most convenient and general way of describing diffusion within zeolite structures. This theory goes a long way to explain the observed various types of dependencies of the Fick diffusivity with sorbate loading. For mixture diffusion within zeolites, the M-S theory provides clues for the development of novel separation techniques that rely on diffusion selectivity. We illustrate this aspect by considering a number of practical applications in membrane permeation and chromatographic separations. (C) 2003 Elsevier B.V. All rights reserved.