Air separation of a cholesteric liquid crystalline triheptyl cellulose/ethyl cellulose (3/97) binary composite membrane prepared from tetrahydrofuran, chloroform, and dichloromethane solutions was studied at different pressure differences across the membrane by the variable volume method. With an increasing pressure difference in the range from 0.06 to 0.42 MPa, the flux and the oxygen concentration of the oxygen-enriched air through the membrane both increase. The air separation capability depends on the boiling point of the membrane-forming solvent and the membrane thickness.