Nafion composite membranes for CO2 separation with good CO2/CH4 separation factors were developed using ionic liquids (ILs) and Nafion. Three ionic liquids containing the 1-n-hexyl-3-methylimidazolium ([hmim]) cation with different anions were characterized by the measurement of the CO2 separation performance using supported ionic liquid membranes in a polysulfone asymmetric support. Two different Nafion membrane-containing ILs for CO2 separation membranes were prepared by the adsorption of IL on a dense and electrospun Nafion/polyethyleneoxide composite membrane in order to improve the stability of the liquid membrane. The electrospun Nafion layer containing polyethyleneoxide was prepared by the electrospinning technique in order to affix [hmim][BF4] (or [hmim][PF6]) on the porous polysulfone asymmetric support. The ideal CO2/CH4 separation factor of electrospun Nafion composite membranes with [hmim][BF4] was consistent with that of [hmim][BF4]. The ideal CO2/CH4 separation factor was 26 for the Nafion composite membranes containing both [hmim][BF4] and [hmim][PF6] in the dense Nafion layer, implying a change in the microdomain of Nafion in the presence of IL and the facilitation effect of ILs for CO2 separation through the IL-swollen Nafion membrane. (C) 2010 Elsevier B.V. All rights reserved.