Using Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation techniques 12 different zeolites were screened to determine which one would yield the best membrane separation selectivity for separation Of CO2 and CH4. The choice of the zeolite has a significant influence on both the sorption and diffusion selectivities. It was found that zeolites CHA and DDR, that have cages separated by narrow windows, yielded the best permeation selectivities. (C) 2007 Elsevier B.V. All rights reserved.