Catalytic water oxidation is a required process for clean energy production based on the concept of artificial photosynthesis. Here, we provide in situ spectroscopic and computational analysis for the closest known photosystem II analog, [Co 4 O 4 ] n+ ([Co 4 O 4 Py 4 Ac 4 ] 0 , Py = pyridine and Ac = CH 3 COO - ), which catalyzes electrochemical water oxidation. In situ extended X-ray absorption fine structure detects an ultrashort, Co IV =O (~1.67 Å) moiety, a crucial intermediate for O-O bond formation. Density function theory analyses show that the intermediate has two Co IV centers and a Co IV =O unit of strong radicaloid character sufficient to support a Co IV =O + H 2 O = Co-OOH + H + transition, where the carboxyl ligand accepts the proton and the bridging oxygen stabilizes the peroxide via hydrogen bonding. The proposed water nucleophilic attack mechanism accounts for all prior spectroscopic evidence on the Co 4 O 4 4+ core. Our results are important for the design and development of efficient water oxidation catalysts, which contribute to the ultimate goal of clean energy from artificial photosynthesis.