Abstract: A process is described for purifying 4,4'-benzophenonedicarboxylic acid, useful in preparing thermoplastic polyesters yielding molded articles having high impact strength, wherein the acid contains aromatic monocarboxylic acid genetic impurity, which comprises the steps of(a) mixing the impure 4,4-benzophenonedicarboxylic acid with aqueous ammonium hydroxide solution, containing about 28 weight percent or less of ammonia in a weight ratio of solution to acid of about 5:1 to 100:1, thereby forming a solution of the diammonium salt of the aromatic dicarboxylic acid;(b) mixing the resultant solution with an inert water-soluble organic liquid which is a non-solvent for the diammonium salt, in a weight ratio of organic liquid to solution not greater than about 20:1, thereby precipitating the diammonium salt of the acid;(c) separating the diammonium salt from the solution; and(d) recovering the aromatic dicarboxylic acid from the diammonium salt.