The incorporation of imidazolium-based ionic liquids into a poly(ether sulfone) (PES) polymeric membrane resulted in a dense and void-free polymeric membrane. As determined through the ideal gas permeation test, the carbon dioxide (CO2) permeation increased about 22 compared to that of the pure PES polymeric membrane whereas the methane (CH4) permeation decreased tremendously. This made the CO2/CH4 ideal separation increase substantially by more than 100. This study highlighted the utilization of imidazolium-based ionic liquids in the synthesis of ionic liquid polymeric membranes (ILPMs). Two different ionic liquids were used to compare the CO2 separation performance through the membranes. The glass-transition temperatures (Tgs) of ILPMs were found to be lower than the Tg of the pure PES polymeric membranes; this supported the high CO2 permeation of the ILPMs due to the increase in PES flexibility caused by ionic liquid addition. The results also draw attention to new trends of ionic liquids as a potential green candidates for future membrane synthesis. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43999. © 2016 Wiley Periodicals, Inc.