The homogenous water oxidation catalysis by [Ru(terpy)(bipy)Cl](+) () and [Ru(terpy)(Me2bipy)Cl](+) () (terpy = 2,2':6',2''-terpyridine, bipy = 2,2'-bipyridine, Me2bipy = 4,4'-dimethyl-2,2'-bipyridine) under the influence of two redox mediators [Ru(bipy)3](2+) () and [Ru(phen)2(Me2bipy)](2+) () (phen = 1,10-phenanthroline) was investigated using Ce(4+) as sacrificial oxidant. Oxygen evolution experiments revealed that mixtures of both - and - produced more molecular oxygen than catalyst alone. In contrast, the combination of mediator and catalyst resulted in a lower catalytic performance of . Measurements of the temporal change in the intensity of a UV transition at 261 nm caused by the addition of four equivalents of Ce(4+) to revealed three distinctive regions-suggested to correspond to the stepwise processes: (i) [Ru(IV)[double bond, length as m-dash]O](2+) → [Ru(V)[double bond, length as m-dash]O](3+); (ii) [Ru(V)[double bond, length as m-dash]O](3+) → [Ru(III)-(OOH)](2+); and (iii) [Ru(III)-(OOH)](2+) → [Ru(II)-OH2](2+). UV-Visible spectrophotometric experiments on the - and - mixtures, also carried out with four equivalents of Ce(4+), demonstrated a faster [Ru(phen)2(Me2bipy)](3+) → [Ru(phen)2(Me2bipy)](2+) reduction rate in - than that observed for the - combination. Cyclic voltammetry data measured for the catalysts and the mixtures revealed a coincidence in the potentials of the Ru(II)/Ru(III) redox process of mediators and and the predicted [Ru(IV)[double bond, length as m-dash]O](2+)/[Ru(V)[double bond, length as m-dash]O](3+) potential of catalyst . In contrast, the [Ru(IV)[double bond, length as m-dash]O](2+)/[Ru(V)[double bond, length as m-dash]O](3+) process for catalyst was found to occur at a higher potential than the Ru(II)/Ru(III) redox process for . Both the spectroscopic and electrochemical experiments provide evidence that the interplay between the mediator and the catalyst is an important determinant of the catalytic activity.