NOVELTY - In separation process for separating at least two substances, a polyimide membrane comprising repeating units of poly(3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride-2,4,6-trimethyl-m-phenylenediamine) polyimide (I), is used.    USE - In separation process i.e. a pervaporation process for separating substances including organic compound such as ethanol, alcohols, phenols, chlorinated hydrocarbons, pyridines and ketones, particularly ethylacetate-ethanol, diethylether-ethanol, acetic acid-ethanol, benzene-ethanol, chloroform-ethanol, chloroform-methanol, acetone-isopropylether, allylalcohol-allylether, allylalcohol-cyclohexane, butanol-butylacetate, butanol-1-butylether, ethanol-ethylbutylether, propylacetate-propanol, isopropylether-isopropanol, methanol-ethanol-isopropanol, and ethylacetate-ethanol-acetic acid from water, sulfur from gasoline or diesel fuels, microorganisms from air or water streams, sulfur-containing molecules from naphtha hydrocarbon streams, in desalination of water by reverse osmosis, and in separation of proteins; and for separating at least a first organic compound from a second organic compound (claimed); used in variety of liquid, gas, and vapor separations such as desalination of water by reverse osmosis, non-aqueous liquid separation such as ethanol/water separations, pervaporation dehydration of aqueous/organic mixtures, CO2/CH4, CO2/N2, H2/CH4, O2/N2, H2S/CH4, olefin/paraffin, iso/normal paraffins separations, and other light gas mixture separations; and used in other applications such as for catalysis, fuel cell applications and transport of gases in a fermenter or a bioreactor.    ADVANTAGE - The polyimide membrane has high permeance and high selectivity for gas separations; has CO2 permeability of greater than or equal to 50 Barrers and single-gas selectivity for CO2/CH4 of greater than or equal to 15 at 50 degrees C under 791 kPa for CO2/CH4 separation; and has UV cross-linkable functional groups and can be used for the preparation of UV cross-linked polyimide membranes having CO2 permeability of greater than or equal to 20 Barrers and single-gas selectivity for CO2/CH4 of greater than or equal to 35 at 50 degrees C under 791 kPa for CO2/CH4 separation.    DETAILED DESCRIPTION - In separation process for separating at least two substances, a polyimide membrane comprising repeating units of poly(3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride-2,4,6-trimethyl-m-phenylenediamine) polyimide compound of formula (I), is used.    X1=group of formula (i) or (ii);    X2=compound (i) or (ii), or group of formula (iii)-(xiii);    Y'=1,3,5-trimethyl-benzen-2,4-diyl, bis(3,5-dimethylphen-4-diyl)methane, 3,3'-dimethyl-biphen-4,4'-diyl, 2,3,5,6-tetramethyl-benzen-1,4-diyl, benzene-1,4-diyl, bis(phen-4-yl)methane, biphen-4,4'-diyl, 1-methyl-benzen-2,4-diyl, oxybis(benzene-4,4'-diyl), 5,5'-oxybis(1,3-dimethylbenzen-2-diyl), or 4,4'-(propane-2,2-diyl)bis((phenoxy-4-diyl)benzene);    n and m=2-500.