TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The hydrophilic diamine is of formula (NH2)2-R-X.R = hydrocarbon radical selected from 6-24C aromatic hydrocarbons and/or 3-12C aliphatic hydrocarbons;X = -OH, -COOH, -SO3H, -NHR1 and/or -NR2R3;R1,R2, R3 = alkyl or aryl.The hydrophilic diamine is about 20-80 mol% of the total of o-tolidine sulfone and hydrophilic diamine. At least one aromatic dianhydride other than 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride may be used. 30-99 mol% of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride is contained in the total of dianhydride monomers. The hydrophilic diamine is 1,3-diamino-2-hydroxypropane, 1,3-diaminobenzene-4-sulfonic acid (HSMPD), 2,2-bis(3-amino-4-hydroxyphenoxy)hexafluoropropane, 3,3'-dihydroxybenzidine, L-lysine, and/or 1,3-diamino-5-benzoic acid. The other aromatic dianhydride is pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenyl tetracarboxylic dianhydride, diphenylsulfone dianhydride and/or bisphenol A dianhydride. Preferably, 1,3-diaminobenzene-4-sulfonic acid is about 40-60 mol% of the total of o-tolidine sulfone and hydrophilic diamine.Preferred Gas Mixture: The gas mixture contacting the membrane may comprise a hydrocarbon solvent (cyclohexane, cycloheptane, cyclooctane, methylcyclohexane, methylcyclopentane, 1,2-dimethylcyclopentane, benzene, toluene and/or xylene) in liquid or gaseous form. Alternatively, the gas mixture comprises carbon dioxide and optionally methane and/or nitrogen. EXAMPLE - A diamine mixture comprising o-tolidine sulfone and 1,3-diaminobenzene-4-sulfonic acid (0.025 moles total), was dissolved in 80-90 ml of N-methyl-2-pyrrolidone (NMP). The aromatic dianhydride or mixture of dianhydrides (0.025 moles) was then added with an additional 10-20 ml of NMP. The solution was allowed to stir for 2 hours at room temperature. 30 ml of o-dichlorobenzene or toluene were then added and the solution was heated and held at 150-180 degreesC for 5-25 hours, for azeotropic removal of the water. The polymer was precipitated into water or methanol, ground, washed three times with methanol and then twice with water. The polymer was air dried at room temperature overnight, then further dried in a vacuum oven at 150-220 degreesC, for at least 2 hours. Inherent viscosity of the polymer (measured at 25 degreesC at 0.5 weight volume % in NMP was 0.69).    The polyimide produced was dissolved in either NMP or m-cresol to form 20% solutions. A film was cast from the solution onto a glass plate at 100-120 degreesC, and dried for 1-2 hours. The film was then removed from the plate, cooled to room temperature and dried further in air overnight. The film was still further dried in a vacuum oven at about 68 kPa at 220 degreesC for 18 hours. A film thickness between 2 x 10-5 and 5 x 10-5 m was thus obtained. The permeabilities at 3.45 MPa and 30 degreesC of pure carbon dioxide and carbon dioxide/methane were respectively 163 barrers and 41.6 barrers/barrers. Fabricated gas separation membranes had high permeability for carbon dioxide (greater than 25 barrers) and carbon dioxide/methane selectivity of more than 40.