Among the many mechanisms for the oxidation of guanine derivatives (G) assisted by transition metal Ru-III and Pt-IV metal ions share basically the same principle. Both Ru-III- and Pt-IV-bound G have highly positively polarized C8-H's that are susceptible to deprotonation by OH-, and both undergo two-electron redox reactions. The main difference is that, unlike Pt-IV, Ru-III is thought to require O-2 to undergo such a reaction. In this study, however, we report that [Ru-III(NH3)(5)(dGuo)] (dGuo = deoxyguanosine) yields cyclic-5'-O-C8-dGuo (a two-electron G oxidized product, cyclic-dGuo) without O-2. In the presence of O-2, 8-oxo-dGuo and cyclic-dGuo were observed. Both [Ru-II(NH3)(5)(dGuo)] and cyclic-dGuo were produced from [Ru-III(NH3)(5)(dGuo)] accelerated by [OH-]. We propose that [Ru-III(NH3)(5)(dGuo)] disproportionates to [Ru-II(NH3)(5)(dGuo)] and [Ru-III(NH3)(4)(NH2-)(dGuo)], followed by a 5'-OH attack on C8 in [Ru-IV(NH3)(4)(NH2-)(dGuo)] to initiate an intramolecular two-electron transfer from dGuo to Ru-IV, generating cyclic-dGuo and Ru-II without involving O-2.