A thermoplastic composition is disclosed that includes a polyaryl ester polymer or copolymer and a phthalone compound according to the formula wherein Z1 represents the atoms necessary to complete a 9- to 13-membered single or fused aromatic ring structure, Z2 represents the atoms necessary to complete a pyridine or quinoline ring, each R1 and each R2 can independently be halogen, an alkyl group, an aryl group, a heterocyclic group, an alkoxy group, an aryloxy group, an aromatic or aliphatic thioether group, an aromatic or aliphatic carboxylic acid ester group, or an aromatic or aliphatic amide group, a is an integer from 0 to 6, b is an integer from 0 to 4, n is 1 or 2, and X is present only if n=2 and is a single bond or a divalent organic radical bonded to the Z1 ring structure through an ether, ketone, or thio linkage.