Soluble, thermally stable phthalazinone poly(aryl ether sulfone ketone)s (PPESKs) containing a carboxyl group in its side chain have been synthesized by the nucleophilic displacement reaction of 4-(4-hydroxylphenyl)-1(2H)-phthalazinone with bis(4-chlorophenyl) sulfone, 4,4'-difluoro-benzophenone, and phenolphthalin. The polymerization reactions were conducted in sulfolane in the presence of K2CO3 to give high molecular weight polymers, which are soluble in solvent such as nitrobenzene and pyridine at room temperature and easily cast into flexible, yellow, and transparent film. The polymers are amorphous with high glass transition temperature. The decomposition temperature of the polymers are >400degreesC, which indicates high thermal stability. The crosslinking reaction of PPESK can occur by using dicyandiamide (Dicy) as curing agent. The apparent energy (DeltaE) is 52.2 kJ/mol and reaction order (n) is close to 1.0. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1111-1114, 2003.