Composite membranes have been prepared by coating a thin layer of polyfluoropropylmethylsiloxane (PTFPMS) on the dense layer of asymmetric polyetherimide (PEI) membranes to plug the defects. The effects of PTFPMS molecular weight and coating solution concentration on the separation performance of the composite membranes have been investigated theoretically and experimentally. The results reveal that PTFPMS 20K (number average molecular weight of 20 K) has smaller molecular size than the defect diameter of the PEI membrane and can plug the defects when its concentration is higher than 5 wt%. With the increase of PTFPMS 20K concentration in the coating solution, the gas permeance of the composite membrane decreases while the separation factor increases. The PEI composite membrane prepared with 20 wt% of PTFPMS 20K exhibits the H(2) permeance of 11.8 GPU (1GPU = 10(-6) cm(3)/(cm(2) s cmHg)), combining with the O(2)/N(2) and H(2)/N(2) separation factors of 6.9 and 107.3, respectively. On the other hand, PTFPMS 1100 K (number average molecular weight of 1100 K) can hardly penetrate into the defects of PEI composite membrane regardless of its concentration in coating solution because of the larger molecular size of FTFPMS 1100 K over the diameter of the defect. Accordingly, the PEI composite membrane prepared with the PTFPMS 1100 K as coating layer shows poor separation performance. Therefore, appropriate match of size between coating material and defect is critical to obtain high separation performance for the PEI composite membrane. More importantly, the solvent resistance experiments indicate that the PTFPMS/PEI composite membrane can maintain its stable separation performance after immersed in heptane and petroleum ether. (C) 2011 Elsevier B.V. All rights reserved.