The ozonation of dissolved Sulfur dioxide is ail important route for sulfate formation, especially in fog and cloud droplets of high pH. However, little is known about the detailed chemical mechanism of this process. We have mapped Out the fate of aqueous SO2 in the presence of ozone by use of density functional theory (DFT) calculations in solution (via the polarized continuum model, PCM), including up to two explicit water molecules. The calculations predict that the hydrolysis of SO2 center dot H2O, although possessing a barrier, is still more energetically favorable than its ozonation. The ozonation of HOSO2- and SO32- proceeds without barriers and gives S(VI) products that are more stable than the reagents by 77.1 and 88.6 kcal/mol, respectively. By comparing our calculated pH dependence of the ozonation kinetics to those determined experimentally, we conclude that, despite a high calculated energy barrier to the ozonation of sulfonate (HSO3-), it is the dominant form of S(IV) in Solutions Of neutral pH and is the species through which ozonation occurs.