The reductions of trans-[Ru(VI)(tpy)(O)2(H2O)2+ or trans-[Ru(VI)(tpy)(O)2(CH3CN)]2+ (tpy is 2,2’:6’,2"-terpyridine) by PPh3, Ph2PCH2CH2PPh2 (dppe), or Ph2PCH2PPh2 (dppm) occur by successive Ru(VI) --> Ru(IV) and Ru(IV)-->Ru(II) oxygen atom transfer steps. The products appear to be five-coordinated diphosphine oxide complexes of Ru(II). They subsequently undergo stepwise solvolysis to give the free diphosphine dioxides and [Ru(II)(tpy)(CH3CN)3]2+. The kinetics of the individual redox steps were studied by stopped-flow/rapid-scan spectrophotometry. For PPh3 as reductant, k(VI/IV)(20-degrees-C, CH3CN) = (2.28 +/- 0.08) X 10(6) M-1 S-1 (DELTAH(double dagger) = 4.2 +/- 0.8 kcal mol-1; DELTAS(double dagger) = -19 +/- 4 eu) and k(IV/II)(20-degrees-C, CH3CN) = (1.04 +/- 0.03) X 10(4) M-1 s-1 (DELTAH(double dagger) = 5.9 +/- 0.5 kcal mol-1; DELTAS double-dagger = -20 +/- 3 eu). With dppe or dppm, Ru(VI) acts as a cis-directed four-electron oxidant. The first step, {Ru(VI) --> Ru(IV)), is first order in both oxidant and diphosphine with k(VI/IV)(20-degrees-C, CH3CN) approximately 4 x 10(8) M-1 s-1 (dppe) to give trans-[Ru(IV)(tpy)(0)(O=P(PPh2)CH2CH2PPh2)(CH3CN)]2+. In acetonitrile with no added water, the subsequent reduction of Ru(IV) to Ru(II) follows first-order kinetics with k(IV/II)(20-degrees-C, CH3CN) = 5 x 10(1) s-1 for either dppe or dppm. By inference, the rate-limiting step is intramolecular isomerization of the remaining oxo group followed by rapid O-atom transfer. In acetonitrile 1.75 M in H2O the initial Ru(IV) product is trans-[Ru(IV)(tpy)(O)(O=P(PPh2)CH2CH2PPh2)(H2O)]2+. The subsequent Ru(IV) --> Ru(II) step is considerably slower, k(IV/II)(20-degrees-C) = (6.20 +/- 0.12) X 10(-2) s-1. This reaction exhibits a substantial inverse solvent isotope effect, k(H2O)/k(D2O)=0.184 +/- 0.02, which arises from the transfer of a single proton on the basis of a mole fraction study. Isomerization is also rate limiting in this case, but the rate-determining step is intramolecular proton transfer.