Under optimal conditions, the N7-coordinated complexes trans-[L(py)(NH3)(4)Ru-III] (L = Guo, dGuo, 1MeGuo) disproportionate to give similar to 50% trans-[Guo(py)(NH3)(4)Ru-II] and a putative Ru-IV species that yields similar to 50% [Gua(py)(NH3)(4)Ru-III]. Disproportionation follows the rate law d[Ru-II]/dt = k(0)[Ru-III] + k(1)[OH-][Ru-III] (for L = Guo : k(0) = 2.9 x 10(-4) s(-1), k(1) = 6.4 M(-1) s(-1)) so that the rate-limiting step in the dominant, hydroxide-dependent pathway is not electron transfer between Ru-III’s, but probably deprotonation of an ammine. Consistent with the ordering of k(1)’s for the ligands (1MeGuo > Guo similar to dGuo > 9MeGua much greater than Gua), ionization of the purine at N1 or N9 slows the disproportionation by suppressing ammine ionization. Activation parameters for k(1) (pH = 11.50) with L = Guo are as follows : Delta H double dagger = 17.4 +/- 0.8 kcal/mol (E(a) = 18.0 +/- 0.8 kcal/mol), and Delta S double dagger = 2.4 +/- 0.1 cal/(mol K). Following disproportionation, the appearance of trans-[Gua(py)(NH3)(4)Ru-III] and free ribose is consistent with general acid hydrolysis of the glycosidic bond induced by Ru-IV, which is subsequently reduced. The rate of appearance of trans-[Gua(py)(NH3)(4)Ru-III] (pH 9.2-11.9) is complicated by purine loss, anation and possibly redox reactions, so that a net hydroxide dependence of approximately [OH-](1/2) was observed. Activation parameters for k(obs) (pH 11.90) with L = Guo are as follows : Delta H double dagger = 24.6 +/- 1.6 kcal/mol (E(a) = 25.2 +/- 1.6 kcal/mol), Delta S double dagger = 8.9 +/- 0.8 cal/(mol K). In the presence of oxygen, trans-[8-OGuo(py)(NH3)(4)Ru-III] was detected as a minor product, but neither 8-oxoguanine nor complexes involving it were observed.