A copolymer with a water-soluble polymer was synthesized from acrylic acid (AA) and 2-(dimethylamino) ethyl methacrylate (DMAEMA) by radical polymerization. The composite membrane was prepared with the DMAEMA-AA copolymer as the top layer and a PS ultrafiltration membrane as the support. The permeation of pure CO, and CH, through the composite membrane was measured. The effects of feed gas pressure, separation layer thickness, temperature and heat treatment on the composite membrane performance were studied. The results show that the composite membranes possess good CO2 permeance and selectivity of CO2 over CH4. The membrane displays a CO2 permeation rate of 6.12 x 10(-7) cm(3) (STP) cm(-2) s(-1) Pa-1 and CO2/CH4 selectivity of 253.1 at 26 degrees C, 1140 Pa of gas pressure.