A mechanism is proposed for the formation of acetone in the OH-initiated atmospheric oxidation of alpha -pinene. In a first step, addition of the OH radical onto the alpha -pinene double bond forms a chemically activated tertiary radical P1OH(dagger). This activated radical can then for a certain fraction break its four-membered ring, leading to a 6-hydroxymenthen-8-yl radical, which is subsequently converted to a 6-hydroxymenthen-8-oxy radical by reaction with O-2 and NO, and elimination of an NO2 molecule. Finally, the 6-hydroxymenthen-8-oxy radical forms acetone by beta C-C bond rupture. For each of these steps, competing reactions are considered, as well as the site and stereospecificity of the reaction itself. To quantify the acetone yield, quantum chemical calculations were combined with RRKM-Master Equation analyses for most of the reactions; other branching ratios were estimated from available literature data. The total yield of acetone was obtained by propagating the relevant product fractions of each step in the mechanism. We find an acetone yield of 8.5%, in good agreement with available experimental data. The uncertainty interval is estimated at 4-16%. It should be emphasized that only the nascent, chemically activated P1OH(dagger); radicals contribute to the crucial ring-breaking isomerization step.