Using a relative rate method, rate constants have been measured for the reactions of OH radicals with 1,2,4,5-tetramethylbenzene, pentamethylbenzene, 2,4,5-trimethylbenzaldehyde, 2,4,5-trimethylphenol and 3-methyl-3-hexene-2,5-dione at 298 +/- 2 K and atmospheric pressure of air. The rate constants obtained (in units of 10(-11) cm(3) molecule(-1) s(-1)) were: 1,2,4,5-tetramethylbenzene, 5.55 +/- 0.34; pentamethylbenzene, 10.3 +/- 0.8; 2,4,5-trimethylbenzaldehyde, 4.27 +/- 0.39; 2,4,5-trimethylphenol, 9.75 +/- 0.98; and 3-methyl-3-hexene-2,5-dione, 9.4 +/- 1.1. The following first-generation products were identified from the OH + 1,2,4,5-tetramethylbenzene reaction in the presence of NO: biacetyl, methylglyoxal, 3-methyl-3-hexene-2,5-dione, and 2,4,5-trimethylbenzaldehyde. The measured molar formation yields for 3-methyl-3-hexene-2,5-dione and 2,4,5-trimethylbenzaldehyde were 45 +/- 9% and 3.3 +/- 0.7%, with that for 3-methyl-3-hexene-2,5-dione being extrapolated to low NO2 concentrations where the OH-1,2,4,5-tetramethylbenzene adducts react only with O-2. Biacetyl appeared to be formed as both a first- and second-generation product, and a first-generation formation yield of 9 +/- 3% was derived. The relative formation yield of methylglyoxal was similar to 0.8 of that for 3-methyl-3-hexene-2,5-dione, indicating that methylglyoxal and 3-methyl-3-hexene-2,5-dione are coproducts. H-atom abstraction from OH + 1,2,4,5-tetramethylbenzene is estimated to account for 3.7 +/- 0.8% of the overall OH radical reaction. On the basis of the current understanding of the mechanism of the OH-aromatic adduct + O-2 reaction, the observed formation of biacetyl indicates that some ipso addition of OH occurs for OH + 1,2,4,5-tetramethylbenzene.