Novel fixed carrier composite membranes were developed by interfacial polymerization with water-soluble trimethylene tetramine (TETA) and hexane-soluble trimesoyl chloride (TMC) on polyethersulfone (PES) supports. A series of the composite membranes were prepared by adopting various heat-treatment time or temperature. Scanning electron microscope (SEM) pictures show that the surface of these membranes is covered with a honeycomb-like structure and the thickness of the skin layers is about 0.2 mu m. The separation performance was tested with CO2/CH4 mixed gas (10/90 by volume) at various feed pressure. The results show that heat-treatment is a successful way in stabilizing membrane performance with pressure. The membrane heat-treated at 65 degrees C for 15 min had a CO2 permeance of 1.33 x 10(-5) cm(3) (STP)cm(-2) s(-1) cmHg(-1) and CO2/CH4 selectivity of 94.1 at feed pressure of 1.1 atm. At 5 atm feed pressure, this membrane had a CO2 permeance of 1.28 x 10-5 cm(3) (STP) cm(-2) s(-1) cmHg(-1) and CO2/CH4 selectivity of 40.5. The optimum heat-treatment conditions were 65-70 degrees C and 10-15 min. (c) 2006 Elsevier B.V. All rights reserved.