Light-driven water oxidation is achieved with the Ru(bpy)(3)(2+)/S2O82- cycle employing the highly active Ir-blue water oxidation catalyst, namely, an Ir-2(IV,IV)(pyalc)(2) mu-oxo-dimer [pyalc = 2-(2'-pyridyl)-2-propanoate]. Ir-blue is readily formed by stepwise oxidation of the monomeric Ir(III) precursor 1 by the photo-generated Ru(bpy)(3)(3+), with a quantum yield phi of up to 0.10. Transient absorption spectroscopy and kinetic evidence point to a stepwise mechanism, where the primary event occurs via a fast photoinduced electron transfer from 1 to Ru(bpy)(3)(3+), leading to the Ir(IV) monomer I-1 (k(1) - 10(8) M-1 s(-1)). The competent Ir-blue catalyst is then obtained from I-1 upon photooxidative loss of the Cp* ligand and dimerization. The Ir-blue catalyst is active in the Ru(bpy)(3)(2+)/S2O82- light-driven water oxidation cycle, where it undergoes two fast photoinduced electron transfers to Ru(bpy)(3)(3+) [with k(Ir-blue), = (3.00 +/- 0.02) x 10(8) M-1 s(-1) for the primary event, outperforming iridium oxide nanoparticles by ca. 2 orders of magnitude], leading to a Ir-2(v,v) steady-state intermediate involved in O-O bond formation. The quantum yield for oxygen evolution depends on the photon flux, showing a saturation regime and reaching an impressive value of phi(O-2) = 0.32 +/- 0.01 (corresponding to a quantum efficiency of 64 +/- 2%) at low irradiation intensity. This result highlights the key requirement of orchestrating the rate of the photochemical events with dark catalytic turnover.