We report a catalytic mechanism for water oxidation in a cobalt oxide cubane model compound, in which the crucial O–O bond formation step takes place by direct coupling between two CoIV(O) metal oxo groups. Our results are based upon density functional theory (DFT) calculations and are consistent with experimental studies of the CoPi water oxidation catalyst. The computation of energetics and barriers for the steps leading up to and including the O–O bond formation uses an explicit solvent model within a hybrid quantum mechanics/molecular mechanics (QM/MM) framework, and captures the essential hydrogen-bonding effects and dynamical flexibility of this system.eni-MIT Solar Frontiers Center (Solar Frontiers Research Program)