Copolyimide derivatives were prepared from two carboxylic dianhydrides [3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) and pyromellitic anhydride (PMDA)] and a single diamine (bis[4-(3-aminophenoxy)phenyl]sulfone [BAPS]) following one-step polymerization. Copolymers could be arranged in sequence through different molar ratios of dianhydride compounds. These polymers were characterized by viscosity, thermal and mechanical properties, solubility, etc. To understand the behavior of the properties, according to the ratio of the dianhydride compound, a copolymer having various properties could be obtained. Further, it was proved that their properties could be determined from the compositions. The solubility of copolyimides with a large molecular weight was moderately improved. (C) 2003 Wiley Periodicals, Inc.