NOVELTY - Producing a polybenzoxazole membrane comprises first fabricating a blend membrane from a mixture of an aromatic polyimide polymer and a polymer containing aromatic sulfonic acid groups, and then converting the blend membrane to a polybenzoxazole membrane by application of heat under an inert atmosphere or under vacuum, where the aromatic polyimide polymer comprises first tetra carbonyl bicyclic repeating unit.    USE - USE u The polybenzoxazole membrane is useful: for gas, vapor and liquid separations (claimed) in chemical, petrochemical, pharmaceutical and allied industries for removing organic vapors from gas streams; in catalysis and fuel cell applications; for separating organic molecules from water, and removing metal and other organic compounds from water; and in olefin cracking applications.    ADVANTAGE - The polybenzoxazole membrane shows improved selectivity for gas, vapor, and liquid separations. The polybenzoxazole membrane provides high processability, selectivity, mechanical stability, and high permeance.    DETAILED DESCRIPTION - Producing a polybenzoxazole membrane comprises first fabricating a blend membrane from a mixture of an aromatic polyimide polymer and a polymer containing aromatic sulfonic acid groups, and then converting the blend membrane to a polybenzoxazole membrane by application of heat under an inert atmosphere or under vacuum, where the aromatic polyimide polymer comprises first tetra carbonyl bicyclic repeating unit of formula (I).    X1 = phenoxybenzen-yl, diphenylmethanon-yl, benzenesulfonylbenzen-yl, (2,2,2-trifluoro-1-phenyl-1-(trifluoromethyl)ethyl)benzen-yl, (1-methyl-1-phenyl-ethyl)benzen-yl, 1-(1-methyl-1-(4-phenoxyphenyl)ethyl)-4-phenoxy-benzen-yl, naphthalene, 1,4-diphenoxybenzen-yl, biphenyl, phenyl group, 2-benzoyloxyethyl benzoate group or phenylsulfanylbenzen-yl (T);    X2 = biphenyl group of formula (a2-b2), phenyl group of formula (c2) or diphenyl group of formula (d2);    R = H or -C(=O)-CH3; and    R1a = -C(CF3)2-, -C(CH3)2-, -S(=O)2-, -O- or -S-.