The nucleophilic addition of SO2, SO2ClF, and SO2F2 to carbenium ions and the nucleophilic addition of SO2 to 1,2-bridged halonium ions are theoretically investigated by using B3LYP/6-311+G(d,p). On the basis of geometric changes in ion-solvent complexes compared to isolated molecules, the theoretical data for the addition of solvent to carbenium ions uniformly agree with experiments by Olah and Donovan. The relative reactivity of carbenium ions (methyl > ethyl > iso-propyl > tert-butyl) follows the familiar trend based on electron demand at the carbenium center. The theoretical data for the addition of SO2 to 1,2-bridged halonium ions with use of similar methods indicate that this addition exhibits a reversed trend on the basis of the electron demand; SO2 adds to 2,2-dimethylethylene chloronium and bromonium ions but does not add to the fluoronium analogue. Furthermore, the addition depends on the stereochemistry of the approaching SO2. When SO2 approaches syn to the halogen atom on the halonium ion, addition is observed. When SO2 is anti, addition is not observed. The reversed reactivity and stereochemistry of the addition of SO2 to halonium ions can be explained by electron donation from the halogen atom to the sulfur atom of the approaching SO2. This sulfur-halogen interaction activates the nucleophilicity of the approaching SO2 and makes a normally unreactive tertiary carbenium carbon susceptible to addition. The theoretical evidence for covalently bound halonium ion-SO2 complexes is discussed in the context of previously reported experimental evidence for the existence of equilibria involving beta-halocarbenium ions.