Bilayer composite membranes suitable for separating air, consisting of poly(4-methylpentene-1) (PMP) thin film as a selective top layer, an ethylcellulose-heptyIcellulose (ECHC) blend thin film as a selective sublayer, and polysulfone as a porous support, were investigated using a constant pressure-variable volume method. By varying operating temperature, pressure, time, as well as stage cut, the membranes were characterized for their oxygen-enriched air (OEA) flux and oxygen concentration in the OEA permeated in a single step. The results show that both the OEA flux and oxygen concentration through the membranes increase with increasing operating pressure. With the increase of operating temperature, the OEA flux increases largely but the oxygen concentration decreases slightly. The oxygen concentration also decreases slightly with the stage cut. On the contrary, the OEA flux decreases and oxygen concentration increases slightly with operating time. It is found that a PMP thin film plays an important role in enhancing the air-separation capability of the membrane. The PMP/ECHC bilayer thin-film composite membrane could enrich the OEA containing 43.6% oxygen at the OEA flux of 5.06 X 10(-4) cm(3) (STP)/s cm(2) with a good performance stability.