A method for preparing asymmetric 3,4'-(hexafluroisopropylidene) dipthalic anhydride (a-6FDA) and asymmetric 3,4'-(methylene)diphthalic anhydride (a-MDPA) from asymmetric 2,3,3',4'-benzophenone dianhydride (a-BTDA). First, a-BTDA is converted to its corresponding N-alkyl or N-phenyl bisimides by reacting with either methylamine or aniline. The sequential addition of two trifluoromethyl (CF.sub.3) groups to the carbonyl unit of a-BTDA produce the corresponding bisimides of asymmetric 6FDA, which is hydrolyzed to the asymmetric 6F-tetracarboxylic acid followed by cyclodehydration to obtain asymmetric 6F-dianhydride (a-6FDA). Separately, the carbonyl unit of bisimides of a-BTDA is reduced to a methylene unit to afford the corresponding bisimides of asymmetric 3,4'-(methylene)diphthalic anhydride, which is hydrolyzed to the asymmetric methylene tetracarboxylic acid followed by cyclodehydration to obtain 3,4'-(methylene)diphthalic anhydride (a-MDPA).