In this work, molecular simulations were performed to evaluate the separation performance of two typical zeolitic imidazolate frameworks (ZIFs), ZIF-68 and ZIF-69, for CO2/N2, CO2/CH 4, and CH4/N2 mixtures. To do this, we first identified a suitable force field for describing CO2, N2, and CH4 adsorption in ZIFs. On the basis of the validated force field, adsorption selectivities of the three mixtures in these ZIFs were simulated then. The results show that ZIF-69 is more beneficial for separating CO2 from CO2-related mixtures than ZIF-68, mainly due to the presence of chlorine atoms in cbIM linkers in the former for the pressures we have considered. The overall separation performances of these two ZIFs for separating the chosen mixtures are comparable to typical MOFs and zeolites. In addition, this work demonstrates that the electrostatic interactions produced by the frameworks are very important for achieving high adsorption separation selectivities in ZIFs, and ideal adsorbed solution theory (IAST) may be applicable to ZIFs. Furthermore, the effect of water on the separation performance of the two ZIFs was also investigated. © 2010 American Chemical Society.