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[Display omitted] • Pebax 1657- [Emim][BF 4 ]/PES composite membranes were prepared by co-casting method. • The gas transport properties of N 2 , CH 4 and CO 2 in the membranes were investigated. • IL loading into the top layer could provide better permeance and CO 2 selectivities. • CO 2 permeance increased more than 150 % in the membrane containing 20 wt. % IL. • For the membrane containing 20 wt% IL, CO 2 /CH 4 and CO 2 /N 2 selectivities increased about 59.22% and 28.64%, respectively. Dual-layer composite membranes composed of Pebax 1657, different 1-Ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF 4 ]) contents and Polyethersulfone (PES) were fabricated by co-casting method, to investigate CO 2 /light gases separation, due to the high affinity of CO 2 to Pebax and the ionic liquid (IL). Gas permeation test, which was performed at 25 °C and pressure range of 4–8 bar, indicated remarkable increment in CO 2 , CH 4 and N 2 permeance through membranes, especially CO 2 , as well as ideal CO 2 /CH 4 and CO 2 /N 2 selectivities, with IL increment. At 6 bar, CO 2 permeance increased 152.97% meanwhile, at 8 bar, CO 2 /CH 4 and CO 2 /N 2 selectivities increased 59.22% and 28.64%, respectively, in the membrane containing 20 wt. % IL, which is the optimum IL content. In addition, CO 2 permeance and selectivity increased with pressure. FTIR, XRD, DSC, SEM, dynamic frequency sweep, contact angle and tensile test results revealed the formation of defect-free dense Pebax/IL top-layer on porous PES sub-layer. IL addition led to formation of new bonds between the IL and Pebax which cause changes in the chain packing and crystallinity of Pebax and fabrication of more amorphous top-layer and more porous sub-layer. These results are confirmed by decrement in the mechanical properties of Pebax/IL casting solutions and composite membranes, by IL increment. [ABSTRACT FROM AUTHOR]