A gas separation membrane is formed from a copolyimide produced by copolymerization of monomers which include o-tolidine sulfone, at least one other hydrophilic diamine and 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride. One or more additional aromatic dianhydrides can optionally be included. The novel membrane exhibits an excellent combination of high selectivity of gases to be separated from a gas mixture, high permeability and strong solvent resistance to hydrocarbon chemicals. The membrane is therefore useful in separating gases from gas mixtures in which vapor or liquid hydrocarbon contaminants are present, for example in the separation of carbon dioxide from methane and/or nitrogen in the purification of natural gas.