Composite membranes for air separation were prepared from a liquid crystal DYC-modified ethyl cellulose (EC) thin film ranging in thickness from 1 to 7 mu m and a porous polyethersulfone support with a thickness of 120 mu m. The effects of DYC/EC (9/91) solution concentration, water, and operating parameters such as temperature, pressure, and time on the air-separation properties of the composite membranes were examined by a constant pressure-variable volume method. The permeate flux and oxygen concentration of the oxygen-enriched air (OEA) through the membranes increase significantly with increasing operating pressure difference. With decreasing casting solution concentration, or with increasing humidity around the membranes or operating temperature, the OEA flux increases greatly while the oxygen concentration sometimes decreases slightly. An increase in the operating time leads to an OEA flux decline, but the oxygen concentration rose when the operating time was varied for 70 hours. However, a further increase of the operating time from 70 to 500 hours does not lead to further changes of the OEA flux and oxygen concentrations. A thin-film composite membrane exhibits a slightly lower oxygen concentration accompanied by a very significant enhancement in the OEA flux and membrane stability compared to a homogeneous dense membrane of the same materials.