The preparation and characteristics of supported carbon membranes from commercial polyimides are reported here. Carbon molecular sieve membranes (CMSM ) with asymmetric and symmetric structures have been obtained in only one casting step by the phase inversion technique. The precursors were polyimide-type polymers available commercially: AllothermA 610-16 (BASF ) and MatrimidA 5218 (Ciba Specialty Chemicals). The preparation method involved coating of the macroporous carbon supports with a polymeric solution and subsequent gellation of the polymeric film by immersion in a coagulant bath. The carbon membranes were obtained by heat treatment (450-700°C ) of the resulting devices under vacuum. The permeability and selectivity data indicate that the resulting carbon membranes possess molecular sieve properties and that they are suitable for the separation of permanent gases of industrial interest (O 2 /N 2 , CO 2 /CH 4 , CO 2 /N 2 , etc.). KaptonA carbon membranes are more permeable than those derived from MatrimidA. The permeability and permselectivity measured at 25°C for KaptonA-derived CMSM were: permeability (CO 2 )=92 Barrer, permselectivity (CO 2 /CH 4 )=16, permselectivity (CO 2 /N 2 )=9; permeability (O 2 )=45 Barrer, permselectivity (O 2 /N 2 )=4. By contrast, the materials obtained from MatrimidA are, in general, less permeable but more selective: permeability (CO 2 )=12 Barrer, permselectivity (CO 2 /CH 4 )=33, permselectivity (CO 2 /N 2 )=15; permeability (O 2 )=5 Barrer, permselectivity (O 2 /N 2 )=6.