In this paper the first variational transition-state rate constant calculation for the OH + CH3COCH3 → P reaction is presented. The potential energy surface has been described by low level calculations at the B3LYP/6-31G* level combined with higher level calculations using the multilevel CBS-RAD technique. Three different reaction pathways have been found: abstraction of an H atom eclipsed to the carbonyl group of acetone, abstraction of an H atom alternated to the carbonyl group of acetone, and addition of an OH molecule to the carbon atom of the carbonyl group of acetone. To take into account the three different kinetic channels, the competitive canonical unified statistical theory has been used to calculate the global rate constant. However, in practice the global rate constant of the acetone + OH reaction turns out finally to be the sum of the eclipsed and alternated abstraction rate constants, leading to a clearly curved Arrhenius plot. The addition-elimination mechanism has an almost negligible contribution to the global rate constant at whatever temperature. The corresponding branching ratio is at most ≈2% and attains even smaller values at the lowest temperature range.