The G2 computational method is applied to the study of the hydroxyl radical oxidation of SO2 to SO3 as well as the hydrolysis of SO2 to H2SO3. A key intermediate in the oxidation process is the HOSO2 radical, which is predicted to have a S-OH bond enthalpy (Delta H-298) of 26.2 kcal/mol, 4.3 kcal/mol lower than the currently accepted value of 30.5 kcal/mol, The radical is characterized by a 2c-2e S-OH bond with an unpaired electron delocalized into the pi* orbital of the SO2 moiety. The hydrolysis of SO2 to H2SO3 was computed with and without a catalytic water. The SO2 . H2O and SO2 . 2H(2)O complexes and transition states are very similar to those computed for SO3 plus water, The uncatalyzed reaction has an activation barrier of 33.9 kcal/mol, which is reduced to 20.0 kcal/mol with one catalytic water. Since the reaction of SO2 with two waters is nearly thermoneutral (4.5 kcal/mol endothermic), the reaction may be more amenable to thermodynamic study compared to the SO3 + 2H(2)O reaction, which is much more exothermic.