A copolyamic acid was synthesized from 3,3’,4,4’-biphenyltetracarboxylic dianhydride (BPDA) as the dianhydride and 4,4’-oxydianiline (4,4’-ODA) and 2,4-diaminotoluene (2,4-DAT) as the diamine and was coated on the outer surface of a porous alumina support tube. The film was imidized and then carbonized by varying reaction period, temperature and atmosphere. Permeances of the BPDA-ODA/DAT carbon membrane were much lower than those of BPDA-ODA carbon membranes. However, the performance of the BPDA-ODA/DAT copolyimide-based membrane was greatly improved by treating in air at temperatures up to 500 degrees C for 1 h, followed by carbonizing in nitrogen at temperatures up to 700 degrees C. Permeance to CO2 for the BPDA-ODA/DAT carbon membrane prepared under optimum conditions was 3x10(-8) mol m(-2) s(-1) Pa-1, and the separation coefficient of CO2 to CH4 was 60 at a permeation temperature of 35 degrees C. These were comparable to the results of carbon membranes prepared from BPDA-ODA polyimide. The micropore structure of the BPDA-ODA/DAT carbon membrane was thus successfully controlled by an optimized combination of oxidation and carbonization after imidization.