The gaseous oxidation of acetone has been studied at temperatures between 250 and 510 $^\circ$C at pressures up to 450 mmHg. In contrast to earlier reports, two oxidation regimes have been discovered, separated by a region where the rate of reaction decreases as the temperature rises. This paper presents the results of a detailed study of the kinetics and products of the reaction in the high-temperature region. The reaction is autocatalytic, and a reaction scheme is proposed which accounts for the principal experimental facts. This scheme involves degenerate chain branching, in which the oxidation of formaldehyde plays an important part.