The photolysis of acetone at 3130 Å has been investigated from 44 to 125°C in the presence of hydrogen bromide for the purpose of obtaining information about aceonte's primary photochemical processes. Hydrogen bromide was found to strongly quench decomposition. It has been proposed that the mechanism of this quenching involves direct trapping of the triplet state by hydrogen bromide. It has been postulated that isopropyl alcohol, a major product of the photolysis, was formed via the triplet state-hydrogen bromide reaction. Coupling of the data obtained in this study with a reinterpretation of the available literature on acetone's fluorescence and phosphorescence permitted Arrhenius factors and activation energies to be evaluated for the triplet and singlet decomposition and the triplet trapping by hydrogen bromide.