The role of cation radical/nucleophile adduct deprotonation equilibria in the reactions of thianthrene cation radical (Th-+) with pyridine and water in acetonitrile solution has been examined using stopped-flow and electrochem-ical techniques. In both reactions reversible nucleophilic attack and adduct formation at a sulfur site on Th-+ is pro-posed as the first step in a general half-regeneration scheme. Rate-determining electron transfer involves reaction between adduct (oxidant) and deprotonated adduct in the case of a protic nucleophile (e.g., water). In the case of an aprotic nucleophile (e.g., pyridine) the rate-determining encounter is between a nonadducted cation radical and adduct with the adduct functioning here as the reducing agent. The formation of the product of both reactions, thianthrene 5-oxide, is discussed in terms of the relative stabilities of the oxidized forms of these cation radical/nucleophile adducts.