Methyl radicals produced by the photolysis of acetone have been allowed to react with oxygen and the kinetics of the reaction studied over a range of conditions from 200 to 400°C. The results of earlier workers have been confirmed, and between 200 and 300 °C, the reaction is of the third order with a rate constant of approximately 10–31 cm6 molecule–2 sec–1 which decreased slightly with increasing temperature. Among the products of the reaction below 300°C, methyl hydroperoxide has been definitely identified and estimated quantitatively. Also, the major decomposition product of methyl hydroperoxide at room temperature is methanol. The formation and further reactions of the methylperoxy radical are discussed in relation to its thermochemistry, with emphasis on the possible role of hydroperoxides in combustion reactions.