The preparation of a composite carbon membrane from a phenolic resin is described. The membrane is formed by a thin microporous carbon layer (thickness: 2 mum) obtained by pyrolysis (700 degreesC, under vacuum) of a phenolic resin film supported on the inner face of a porous alumina tube. The separation characteristics of the resulting carbon membranes were analysed from permeation experiments with pure gases of different molecular size (He, CO2, O-2, N-2 and CH4) and separation of binary gas mixtures O-2-N-2 and CO2-CH4. An almost defect-free carbon membrane is obtained in only one casting step, The effective micropore size was estimated to be around 4.4 Angstrom. The prepared carbon membranes have demonstrated to be effective for separating gas mixtures such as O-2-N-2 (O-2 permeance: 100 Barrer; O-2-N-2 separation factor: 12) and CO2-CH4 (CO2 permeance: 400 Barrer; CO2-CH4 separation factor: 150). The oxidation of the phenolic resin film with air at temperatures ranging from 150 to 300 degreesC improves the gas permeance and originates a decrease in the permselectivity.