A new dinuclear Ru-OH₂ complex has been prepared and serves as a water oxidation catalyst (see figure; O?red, N?lavender, Ru?pink). Structural and kinetic analyses, as well as reactivity tests provide compelling evidence, showing that the crucial O-O bond-formation steps occur through a bimolecular I2M mechanism. Furthermore, this bimolecular interaction is demonstrated for the first time using ¹⁸O-labeling experiments.