The homogeneous catalysis of water oxidation by transition-metal complexes has experienced spectacular development over the last five years. Practical energy-conversion schemes, however, require robust catalysts with large turnover frequencies. Herein we introduce a new oxidatively rugged and powerful dinuclear water-oxidation catalyst that is generated by self-assembly from a mononuclear catalyst during the catalytic process. Our kinetic and DFT computational analysis shows that two interconnected catalytic cycles coexist while the mononuclear system is slowly and irreversibly converted into the more stable dinuclear system: an extremely robust water-oxidation catalyst that does not decompose over extended periods of time. In for the long haul: The transformation of a highly active mononuclear ruthenium-aqua water-oxidation catalyst into a dinuclear complex during oxygen-evolution catalysis led to the coexistence of two different catalytic cycles in solution (see picture; Ru pink, N blue, O red). The dinuclear species was much more robust than its mononuclear counterpart and remained an active catalyst for water oxidation for extended periods of time. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.