Products of the gas-phase reactions of the OK radical with p-xylene and 1,2,3- and 1,2,4-trimethylbenzene have been measured by gas chromatography in the presence of varying concentrations of NO2. Our product analyses show that the ring-cleavage products 2,3-butanedione (from 1,2,3- and 1,2,4-trimethylbenzene) and 3-hexene-2,5-dione (from p-xylene and 1 4-trimethylbenzene) exhibit a dependence of their formation yields on the NO2 concentration, with higher yields from the reactions of the OH-aromatic adducts with O-2 than from their reactions with NO2. Furthermore, our data show that these ring-cleavage products are primary products of the OH-aromatic adduct reactions. Formation yields extrapolated to zero NO2 concentration should be applicable to ambient atmospheric conditions (provided that there is sufficient NO that peroxy radicals react dominantly with NO), and are from p-xylene, p-tolualdehyde, 0.0706 +/- 0.0042 (independent of NO2 concentration), 2,5-dimethylphenol, 0.138 +/- 0.016 (independent of NO2 concentration), and 3-hexene-2,5-dione, 0.32 (extrapolated); from 1,2,3-trimethylbenzene, 2,3-butanedione, 0.52 (extrapolated); and from 1,2,4-trimethylbenzene, 2,3-butanedione, 0.10 (extrapolated) and 3-hexene-2,5-dione, 0.31 (extrapolated). Our formation yields of 3-hexene-2,5-dione from p-xylene and 1,2,4-trimethylbenzene are similar to those reported for glyoxal from p-xylene and of methylglyoxal from 1,2,4-trimethylbenzene and therefore suggest that these are coproducts, as expected from reaction schemes presented here.