It takes two: A new dinuclear Ru-OH 2 complex has been prepared and serves as a water oxidation catalyst (see figure; Ored, Nlavender, Rupink). 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. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.