Aromatic hydrocarbons are important constituents of vehicle exhaust and of nonmethane organic compounds in ambient urban air. We used a derivatization technique with methane positive chemical ionization gas chromatography/mass spectrometry to investigate the carbonyl products formed from the OH radical-initiated reactions of toluene, the xylenes, and the trimethylbenzenes. Characteristic diderivatized molecular ions of dicarbonyl products were obtained. Consistent with previous studies, the 1,2-dicarbonyls glyoxal, methylglyoxal, and biacetyl were observed, as were all but one of the possible unsaturated 1,4-dicarbonyl coproducts. Unsaturated 1,4-diketones had formation yields similar to their potential coproduct 1,2-dicarbonyls. However, apart from HC(O)CHCHCHO, unsaturated 1,4-dialdehydes and keto-aldehydes were generally observed in lower yield than their potential 1,2-dicarbonyl coproducts.