The preparation, characterization, and gas permeation properties of novel composite membranes containing polyimide (PI), liquid sulfolane (SF), and zeolite (ZSM-5) were investigated to address the interface defects between the PI and the zeolite. The free-standing composite membranes were prepared by the solvent casting method. The gas permeability of the PI+ZSM-5 membrane was higher than that of PI, whereas its gas selectivity was significantly reduced, suggesting that these results are attributed to the interface defects. The CO2 selectivity of PI+ZSM-5+SF was higher than those of the PI+ZSM-5 membranes because of the introduction of liquid SF into the interface defects. Furthermore, liquid SF enhanced the CO2/H2 selectivity near the recent upper bound. Therefore, the use of liquid SF could be an effective approach to preventing interface defects and increasing the CO2 selectivity, particularly for CO2/H2. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013