Pentaammineruthenium(III) complexes of deoxyinosine (dIno) and xanthosine (Xao) ([Ru(III)(NH(3))(5)(L)], L is dIno, Xao) in basic solution were studied by UV-vis spectroscopy, liquid chromatography/electrospray ionization mass spectrometry, and high-performance liquid chromatography. Both Ru(III) complexes disproportionate to Ru(II) and Ru(IV). Disproportionation followed the rate law d[Ru(II)]/dt = (k (o) + k (1)[OH(-)])[Ru(III)]. k (o) and k (1) of disproportionation at 25 °C were 2.1 (±0.1) × 10(-3) s(-1) and 21.4 ± 3.2 M(-1) s(-1), respectively, for [Ru(III)(NH(3))(5)(dIno)], and 3.5 (±0.7) × 10(-4) s(-1) and 59.7 ± 3.6 M(-1) s(-1), respectively, for [Ru(III)(NH(3))(5)(Xao)]. The [Ru(III)(NH(3))(5)(Xao)] complex disproportionates at a faster rate than [Ru(III)(NH(3))(5)(dIno)] owing to the stronger electron-withdrawing effect of exocyclic oxygen in Xao. The activation parameters ΔH (‡) and ΔS (‡) for k (1) of [Ru(III)(NH(3))(5)(dIno)] were 80.2 ± 15.2 kJ mol(-1) and 47.6 ± 9.8 J K(-1) mol(-1), respectively, indicating that the disproportionation of Ru(III) to Ru(II) and Ru(IV) is favored owing to the positive entropy of activation. The final products of both complexes in basic solution under Ar were compared with those under O(2). Under both conditions [Ru(NH(3))(5)(8-oxo-L)] was produced, but via different mechanisms. In both aerobic and anaerobic conditions, the deprotonation of highly positively polarized C8-H of Ru-L by OH(-) initiates a two-electron redox reaction. For the next step, we propose a one-step two-electron redox reaction between L and Ru(IV) under anaerobic conditions, which differentiates from Clarke's mechanism of two consecutive one-electron redox reactions between L, Ru(III), and O(2).