This paper presents a method for the separation of CO2/CH4 and CO2/N-2 by the use of disk supported carbon membranes derived from polyimide. The supported polyimide membranes were carbonized under controlled gas flow (three different atmospheres: N-2, Ar and He) at different carbonization temperatures from 600 to 900 degrees C. The carbon membrane fabricated at various carbonization temperatures and atmospheres, showed improvement in gas permeation performance. The gas permeability increased with the increase of carbonization temperature due to densification of the micropores in the carbon membranes, leading to enhanced gas permselectivity. All carbon membrane samples carbonized under three different atmospheres at 800 degrees C surpassed the polymeric CO2/CH4 upper bound. Pure-gas permselectivity reached 98 for the carbon membrane carbonized at 800 degrees C under He gas flow, with CO2 permeability of about 120 Barrer, becoming the most selective carbon membrane for CO2/CH4 separation.