The oxidation of water catalyzed by [Ru(tpy)(bpy)(OH2)](ClO4)(2) (1; tpy = 2,2';6 '',2 ''-terpyridine; bpy = 2,2'-bipyridine) is evaluated in different acidic media at variable oxidant concentrations. The observed rate of dioxygen evolution catalyzed by 1 is found to be highly dependent on pH and the identity of the acid; e.g., d[O-2]/dt is progressively faster in H2SO4, CF3SO3H (HOTf), HClO4, and HNO3, respectively. This trend does not track with thermodynamic driving force of the electron-transfer reactions between the terminal oxidant, (NH4)(2)[Ce(NO3)(6)] (CAN), and Ru catalyst in each of the acids. The particularly high reactivity in HNO3 is attributed to the NO3- anion: (i) enabling relatively fast electron-transfer steps; (ii) participating in a base-assisted concerted atom-proton transfer process that circumvents the formation of high energy intermediates during the O-O bond formation process; and (iii) accelerating the liberation of dioxygen from the catalyst. Consequently, the position of the rate-determining step within the catalytic cycle can be affected by the acid medium. These factors collectively contribute to the position of the rate-determining step within the catalytic cycle being affected by the acid medium. This offering also outlines how other experimental issues (e.g., spontaneous decay of the Ce(IV) species in acidic media; CAN/catalyst molar ratio; types of catalytic probes) can affect the Ce(IV)-driven oxidation of water catalyzed by homogeneous molecular complexes.