The preparation of a composite carbon membrane for separation of gas mixtures is described. The membrane is formed by a thin microporous carbon layer (thickness, 2 mu m) obtained by pyrolysis of a phenolic resin film supported over a macroporous carbon substrate (pore size, 1 mu m; porosity, 30%). The microporous carbon layer exhibits molecular sieving properties and it allows the separation of gases depending on their molecular size. The micropore size was estimated to be around 4.2 Angstrom. Single and mixed gas permeation experiments were performed at different temperatures between 25 degrees C and 150 degrees C, and pressures between 1 and 3.5 bar. The carbon membrane shows high selectivities for the separation of permanent gases like O-2/N-2 system (selectivity approximate to 10 at 25 degrees C). Gas mixtures like CO2/N-2 and CO2/CH4 are successfully separated by means of prepared membranes. For example, the membrane prepared by carbonization at 700 degrees C shows at 25 degrees C the following separation factors: CO2/N(2)approximate to 45 and CO2/CH(4)approximate to 160.