The kinetics and mechanism of reduction of trans-[Ru-VI(tpy)(O)(2)(L)](2+) (L is H2O or CH3CN; tpy is 2,2':6',2 " terpyridine) by benzyl alcohol have been studied in water and acetonitrile. The reactions are first order in alcohol and complex in both solvents and,give benzaldehyde as the sole oxidation product. In acetonitrile, sequential Ru-VI --> Ru-IV and Ru-IV --> Ru-II' steps occur. As shown by FTIR and UV-visible measurements, Ru-II' solvolyzes to give [Ru-II(tpy)(CH3CN)(3)](2+) and benzaldehyde. With O-18-labeled Ru-VI, similar to 50% of the label ends up in the aldehyde product for both the Ru-VI-->Ru-IV and Ru-IV-->Ru-II steps as shown by FTIR. In water, Ru-VI-->Ru-IV reduction is followed by rapid dimerization by mu-oxo formation. Kinetic parameters for the individual redox steps in 0.1 M HClO4 at 25 degrees C are k(VI-->IV) = 13.3 +/- 0.8 M-1 s(-1) (Delta H-double dagger = 11.4 +/- 0.2 kcal/mol, Delta S-double dagger = -15.0 +/- 1 eu, k(H)/k(D) 10.4 for alpha,alpha-d(2) benzyl alcohol). In CH3CN at 25 degrees C, k(VI-->IV) = 67 +/- 3 M(-1)s(-1) (Delta H-double dagger = 7.5 +/- 0.3 kcal/mol, Delta S-double dagger = -33 +/- 2 eu, k(H)/k(D) = 12.1) and k(IV-->II') = 2.4 +/- 0.1 (Delta H-double dagger = 5.1 +/- 0.3 kcal/mol, Delta S-double dagger = -47 +/- 2 eu,k(H)/k(D) = 61.5). On the basis of the O-18 labeling results in CH3CN, the O atom of the oxo group transfers to benzyl alcohol in both steps. Mechanisms are proposed involving prior coordination of the alcohol followed by O insertion into a benzylic C-H bond.