The comproportionation reaction between [Ru-IV(bpy)(2)(py)(O)](2+) and [Ru-II(bpy)(2)(py)(OH2)](2+) in acetonitrile (CH3CN) to form 2 equiv of [Ru-III(bpy)(2)(py)(OH)(2+) (bpy = 2,2’-bipyridine, py = pyridine) was investigated by means of rapid-scan, stopped-flow kinetics, yielding the rate constant k(com) = (4.07 +/- 0.13) x 10(3) M(-1) s(-1) at 25 degrees C. Under the same conditions in 1% W/V H2O or D2O in CH3CN the rate constants were (3.10 +/- 0.12) x 10(3) M(-1) s(-1) and (2.13 +/- 0.02) x 10(2) M(-1) s(-1), respectively. The solvent isotope effect k((H2O))/k((D2O)) = 14.6 +/- 0.7 provides evidence for proton-coupled electron transfer. The long term instability of [Ru-III(bpy)(2)(py)(OH)](2+) in CH3CN was investigated by following the changes in the UV-vis spectrum over periods up to 2 x 10(5) s. Principal factor analysis of the spectral changes by singular value decomposition revealed the presence of four colored components during the reaction. The application of global kinetic analysis methods allowed the data to be fit to a model involving initial disproportionation to Ru-IV=O2+ and Ru-II-OH22+ (k(disp) = 81 +/- 8 M(-1) s(-1), 25 degrees C), followed by irreversible substitution of the aqua complex by CH3CN to give [RU(II)(bpy)(2)(py)(NCCH3)](2+) (k(solv) = (1.66 +/- 0.02) x 10(-3) s(-1), 25 degrees C). Further reaction of Ru-IV=O2+ with solvent or impurities also occurred to give [Ru-II(bpy)(2)(py)(NCCH3)](2+) (k similar to 5 x 10(-6) s(-1), 25 degrees C). An independent study of the loss of [Ru-IV(bpy)(2)(py)(O)](2+) in CH3CN revealed that Ru-III-OH2+ was formed as an intermediate during this reaction. The rate constants from the global kinetic analysis also provided an estimate of the equilibrium constant for comproportionation, K-com = 50 +/- 5 in CH3CN. The dependence of k(solv) on [H2O] for solvolysis of the aqua complex by CH3CN is consistent with a dissociative (D) mechanism. The competition ratio ratio for capture of the 5-coordinate intermediate by H2O or CH3CN is K-AQ/K-AN = 18.4 +/- 0.6 at 25 degrees C.