Based on density functional theory calculations we present a study of the gaseous oxidation of SO2 to SO3 by an anionic O3−(H2On cluster, n=0–5. The configurations of the most relevant reactants, transition states, and products are discussed and compared to previous findings. Two different classes of transition states have been identified. One class is characterized by strong networks of hydrogen bonds, very similar to the reactant complexes. The other class is characterized by loose structures of hydration water and is stabilized by high entropy. At temperatures relevant for atmospheric chemistry, the most energetically favorable class of transition states vary with the number of water molecules attached. A kinetic model is utilized, taking into account the most likely outcomes of the initial SO2O3−(H2O)n collision complexes. This model shows that the reaction takes place at collision rates regardless of the number of water molecules involved. A lifetime analysis of the collision complexes supports this conclusion. Hereafter, the thermodynamics of water and O2 condensation and evaporation from the product SO3−O2(H2O)n cluster is considered and the final products are predicted to be O2SO3− and O2SO3−(H2O)1. The low degree of hydration is rationalized through a charge analysis of the relevant complexes. Finally, the thermodynamics of a few relevant reactions of the O2SO3− and O2SO3−(H2O)1 complexes are considered.