Composite membranes composed of glassy fluorine-containing polyimide (PI) containing up to 81 wt% of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][Tf2N]) ionic liquid (IL) were prepared by a casting method. With increased IL content to 51 wt%, a microsized domain of IL was formed in the PI matrix. This domain continuously expanded with increased IL content based on the SEM images. The gas permeability of composite membranes was determined at 35 degrees C. With increasing 35 wt% IL, the gas permeability of PI+IL membranes linearly decreased, whereas the gas permeability increased from 51 to 81 wt% IL content. This increase in gas permeability was attributed to the increase in gas diffusivity caused by the formation of IL domains and the polyimide membrane plasticization effect. The composite membranes with high IL content showed good CO2/light gas (e.g., CO2/H-2 and CO2/O-2) separation performance. The gas selectivity at 0 degrees C significantly improved and was located near the recently upper bound due to the increase in the solubility selectivity. (C) 2012 Elsevier B.V. All rights reserved.