The oxidation of benzaldehyde and several of its derivatives to their carboxylic acids by cis-[Ru-IV(bpy)(2)(py)(O)](2+) (Ru-IV=O2+; bpy is 2,2'-bipyridine, py is pyridine), cis-[Ru-III(bpy)(2)(py)(OH)](2+) (Ru-III-OH2+), and [Ru-IV(tpy)(bpy)(O)](2+) (tpy is 2,2':6',2"-terpyridine) in acetonitrile and water has been investigated using a variety of techniques. Several lines of evidence support a one-electron hydrogen-atom transfer (HAT) mechanism for the redox step in the oxidation of benzaldehyde. They include (i) moderate k(C-H)/k(C-D) kinetic isotope effects of 8.1 +/- 0.3 in CH3CN, 9.4 +/-0.4 in H2O, and 7.2 +/- 0.8 in D2O; (ii) a low k(H2O/D2O) kinetic isotope effect of 1.2 +/- 0.1; (iii) a decrease in rate constant by a factor of only similar to 5 in CH3CN and similar to 8 in H2O for the oxidation of benzaldehyde by cis-[RuIII(bpy)(2)(py)(OH)](2+) compared to cis-[Ru-IV(bpy)(2)(py)(O)](2+); (iv) the appearance of cis-[Ru-III(bpy)(2)(py)(OH)](2+) rather than cis-[Ru-III(bpy)(2)(py)(OH2)](2+) as the initial product; and (v) the small p value of -0.65 +/- 0.03 in a Hammett plot of log k vs sigma in the oxidation of a series of aldehydes. A mechanism is proposed for the process occurring in the absence Of 02 involving (i) preassociation of the reactants, (ii) H-atom transfer to Ru-IV=O2+ to give Ru-III-OH2+ and PhCO, (iii) capture of PhCO by Ru-III-OH2+ to give Ru-II-OC(O)Ph+ and H+, and (iv) solvolysis to give cis-[Ru-II(bpy)(2)(py)(NCCH3)](2+) or the aqua complex and the carboxylic acid as products.