Selectivity for gas separation from a binary mixture in metal organic framework (MOF) materials is identified as arising from a window effect. Our initial analysis based on geometrical models shows that the selectivity, which is related to shape and size of the gas molecules, can be tuned by the window size. Classical molecular dynamics are then used to evaluate CO2/N-2 selectivities in MOF-5 derivatives with various window structures. The results verify that the size/shape of molecular windows can change diffusion rates and consequently result in selectivity enhancement. The effect may effectively boost the overall performance of diffusion-driven separation in MOFs. (C) 2012 Elsevier B.V. All rights reserved.