We examine the mechanism of electron transfer between [Ru(bpy)(3)](2+) and [S(2)O(8)](2-) in aqueous solutions using time-dependent density functional and transition charge density model levels of theory. The calculations support the existence of a short-lived, optically bright S1 state that decays to lower-lying triplet states T1 and T2. The T1 state leads to the charge transfer products, that is, [Ru(bpy)(3)](3+) + SO(4)(-*) + SO(4)(2-). It was shown that photoinduced (in the 420-520 nm wavelength range) electron transfer between [Ru(bpy)(3)](2+) and [S(2)O(8)](2-) may proceed via both the "unimolecular" and "bimolecular" pathways, in agreement with the previous experimental findings. This distinction arises on the basis of a weak interaction between the two reactants. Analysis of excited electronic states and their spin-orbit mixing suggests that a photon excites a MLCT S1 state of [Ru(bpy)(3)](2+)*/{[Ru(bpy)(3)](2+)...[S(2)O(8)](2-)}* that converts to the lower-lying T2 state via spin-orbit interaction, which is an intermediate in the S1 --> T2 --> T1 chain. Once the system has converted to T1, it can evolve toward charge transfer from [Ru(bpy)(3)](2+)* to [S(2)O(8)](2-) via elongation of the peroxo O-O bond that brings the "zero order" states of the [Ru(bpy)(3)](2+)* and [S(2)O(8)](2-) fragments into (near) resonance. An exciton interaction model in combination with the transition charge density model provides excellent agreement with TD-DFT and allows us to obtain insight into the electron transfer process. The steady-state luminescence studies of the quenching of [Ru(bpy)(3)](2+)* in aqueous solutions by [S(2)O(8)](2-) at pH 7.2 and 20 mM sodium phosphate buffer, the exact conditions used in our previous study of water oxidation [Geletii, Y. V.; Huang, Z. Q.; Hou, Y.; Musaev, D. G.; Lian, T. Q.; Hill, C. L. J. Am. Chem. Soc. 2009, 131, 7522-7523], show that 61% of the [Ru(bpy)(3)](2+) forms a ground state ion-pair complex [Ru(bpy)(3)](2+)...[S(2)O(8)](2-), and the unimolecular ET rate is 2.5 times faster than for the bimolecular process. The concentration of the ion-pair complexes decreases at higher sodium phosphate buffer.