The transport properties of ionic liquid (IL)/sulfonated polyimide (SPI) composite membranes for CO2 separation were explored in relation to their nanostructures. 1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C(4)mim][NTf2]), 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim]PF6), and 1-butyl-1-methylpyrrolidinium bis (trifluoromethanesulfonyl) amide ([P-14] [NTf2]) were selected as the ILs. These composite membranes enable favorable CO2 separation and superior mechanical properties. SPI is plasticized by the IL, which induces a decrease in the elastic modulus; however, due to the formation of bicontinuous nanostructures of IL-rich and SPI-rich phases, a modulus of >10 MPa is retained even with the incorporation of 75 wt % IL. [C(4)mim] [NTf2] and [C(4)mim]PF6 exhibit stronger plasticization effects on SPI than [P-14] [NTf2]. The diffusion coefficients of CO2 and N-2, which were measured by the time-lag method, abruptly increase with increasing IL content in the [C(4)mim][NTf2]/SPI composite membranes, which coincided with a change from an isolated to a continuous IL-rich phase structure. On the other hand, the selective solubility of CO2 in the [C(4)mim][NTf2]/SPI composite membranes exhibits no relation with the IL content. Consequently, the permeation coefficient (P-CO2) of the membranes increases with increasing IL content without decreasing the separation factor (alpha(CO2/N2)), showing P-CO2 = 431 barrer and alpha(CO2/N2) = 30 at 30 degrees C.