Cage-type micro-porous materials such as LTA, CHA, SAPO-34, DDR, ERI, ZIF-7, and ZIF-8 have significant potential for use in membrane technologies for CO2 capture. The permeation selectivities are governed by a combination of adsorption and diffusion selectivities, each of which can be separately manipulated. In this study we have investigated the characteristics of CO2/CH4 mixture diffusion in ZIF-8 by monitoring the uptake within crystals using infra-red microscopy ( IRM). The uptake profiles were fitted to determine the "effective" Fick diffusivities of the individual components in the mixture. The diffusivity measurements, conducted for a range of loadings, clearly demonstrate that the preponderance of CO2 at the window regions of ZIF-8 hinders the inter-cage transport of partner CH4 molecules. Such hindering effects serve to enhance the CO2/CH4 permeation selectivities across ZIF-8 membranes for high mixture loadings within the membrane above values anticipated on the basis of unary permeances. (C) 2012 Elsevier B.V. All rights reserved.