Relative rate constants have been measured for the reactions of eight pairs of organic compounds with the reactive species formed in the reaction of ozone with 2-methylbut-2-ene. A total of eight compounds were studied, including three alkanes, two alkenes, two conjugated alkenes and one aromatic hydrocarbon. With the exception of one pair of compounds (cyclohexane/2-methylpropane), the relative rate constants obtained are consistent with the assumption that the reactive intermediate is the hydroxyl radical, OH. The origin of the anomalous result for the cyclohexane/2-methylpropane pair is not clear. ¿ 1998 American Geophysical Union