The gas phase reactions of hydroxyl radicals with o-Xylene and m-Xylene were studied in a continuous stirred tank reactor. Gas and aerosol products accounted for 65 to 85 percent of the reacted carbon. Approximately 19 and 10 percent of the original o-Xylene and m-Xylene oxidation occurred through methyl hydrogen abstraction by OH, primarily leading to methylbenzyl nitrates and tolualdehydes. The remaining mass reacted through the OH addition pathway forming dimethylphenols, nitro-dimethylphenols, nitroxylenes and stable products resulting from further reaction of metastable 02-OH-adducts (including alpha-dicarbonyls, methyl-p-benzoquinones, CO and formaldehyde). For o-Xylene the ratio of reaction rate constants for formation of nitroxylenes versus dimethylphenols was estimated to be 59,000, while the same value for m-xylene was only about 10,000. The ratios of the dimethylphenol formation rates to the effective oxygen addition rates were found to be greater than or equal to 0.15 for o-Xylene and 0.27 for m-Xylene. These vary significantly from the approximations used in current photochemical kinetics models.