In phosphorothioate-containing dsDNA oligomers (S-oligomers), one of the two nonbridging oxygen atoms in the phosphate moiety of the sugar-phosphate backbone is replaced by sulfur. In this work, electron spin resonance (ESR) studies of one-electron oxidation of several S-oligomers by Cl-2(center dot-) at low temperatures are performed. Electrophilic addition of Cl-2(center dot-) to phosphorothioate with elimination of Cl- leads to the formation of a two-center three-electron sigma(2)sigma*(1)-bonded adduct radical (-P-S center dot Cl). In AT S-oligomers with multiple phosphorothioates, i.e., d[ATATAsTsAsT](2), -P-S-Cl reacts with a neighboring phosphorothioate to form the sigma(2)sigma*(1)-bonded disulfide anion radical ([-P-S-S-P-](-)). With AT S-oligomers with a single phosphorothioate, i.e., d[ATTTAsAAT](2), reduced levels of conversion of -P-SCl to [-P-S-S-P-](-) are found. For guanine-containing S-oligomers containing one phosphorothioate, -P-S,Cl results in one-electron oxidation of guanine base but not of A, C, or T, thereby leading to selective hole transfer to G. The redox potential of -P-S-Cl is thus higher than that of G but is lower than those of A, C, and T. Spectral assignments to -P-S-Cl and [-P-S-S-P-](-) are based on reaction of Cl-2(center dot-) with the model compound dfisopropyl phosphorothioate. The results found for d[TGCGsCsGCGCA](2) suggest that {-P-S-S-P-](-) undergoes electron transfer to the one-electron-oxidized G, healing the base but producing a cyclic disulfide-bonded backbone with a substantial bond strength (50 kcal/mol). Formation of -P-S-Cl and its conversion to [-P-S-S-P-](-) are found to be unaffected by O-2 and this is supported by the theoretically calculated electron affinities and reduction potentials of [-P-S-S-P-] and O-2.