A systematic computational study was performed in this work to investigate the dispersion behaviors of ionic liquids (ILs) in metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) as well as the separation performance of the resulting composites for CO2/CH4 and CO2/N-2 mixtures. Five MOFs and eight COFs with diverse pore structures and chemical properties were selected as the supporters for 1-n-butyl-3-methy limidazolium thiocyanate [BMIM][SCN]. The results show that stronger Coulombic interactions contributed from the frameworks of MOFs can lead to better dispersion of the IL molecules in their pores compared with COFs. The gas separation performance can be significantly enhanced by introducing [BMIM]ISCN] into MOFs and COFs, and MOFs can be considered as better support materials for ILs. Better dispersion of the IL in a given support material will induce greater enhancement on the separation performance of the composite, and such phenomenon is more evident for CO2/CH4 mixture compared with the CO2/N-2 system. The IL molecules are more inclined to aggregate in the 2D-COFs and MOFs with 1D pore structures. However, they are more dispersive in the materials with 3D pore structures as the supporters, leading to a more evident improvement on the separation performance. This work also shows that using the materials containing strong adsorption sites like coordinatively unsaturated metal sites as the supporters for ILs cannot achieve significant enhancement on the gas separation performance of the composites. (C) 2015 Elsevier Ltd. All rights reserved.