Acetone is a significant pollutant in contaminated groundwaters and industrial effluents. It can be treated by the UV/H2O2 process but only slowly. This study aims to understand the degradation mechanism and hence the reasons for slow treatment. The degradation of acetone was carried out in a UV reactor in the presence of ∼16 mM H2O2 such that most of the UV was absorbed by H2O2. The decay of acetone was followed by gas chromatography, and the generation of intermediates (identified as acetic, formic, and oxalic acids) was followed by ion chromatography. Measurement of the total organic carbon indicated a complete carbon balance throughout the reaction ending in mineralization. A kinetic model, based on an assumed mechanism, was developed that generated a profile of reactants and intermediates in agreement with the experimental data, including the pH profile. The initial concentrations of acetone and hydrogen peroxide strongly affect the initial rate of acetone degradation, but no pH effect was observed in the range of 2−7. It is concluded that acetone treats slowly because intermediates build up to such a concentration that they compete significantly for hydroxyl radicals and also because the mechanism appears to involve some degree of acetone recycling.