The oxidation of SO2 by nonmetal oxoanions in the gas phase is investigated in an experimental and theoretical study of the structure of the species involved and the reaction kinetics and mechanism. SO3, SO3.− and SO4.− are efficiently produced by reaction of OnXO− anions (X=Cl, Br, and I; n=0 and 1) with SO2; XO− ions mainly react to give SO3 by oxygen-atom transfer, whereas OXO− ions mainly give SO3.− by oxygen-anion transfer. On descending the halogen group from chlorine to iodine, the SO3/SO3.− ratio decreases and increases for reactions involving XO− and OXO− anions, respectively, whereas the formation of SO4.− is particularly significant with OIO−. Kinetic factors play a major role in the reactions of OnXO−, depending on the halogen atom and its oxidation state.