The consumption of labeled methylphenylcarbinol during the liquid-phase oxidation of ethylbenzene at 120" has been investigated. Oxidation of the carbinol yielded only acetophenone, but most of the latter was formed directly, by the termination reaction and by decomposition of hydroperoxide molecules. Use of the modified kinetic isotope method permitted quantitative calculation of rates of formation of alcohol and ketone molecules by various routes.