Recently it was reported that copper bicarbonate/carbonate complexes are good electro‐catalysts for water oxidation. However, the results did not enable a decision whether the active oxidant is a CuIII or a CuIV complex. Kinetic analysis of pulse radiolysis measurements coupled with DFT calculations point out that CuIII(CO3)n3−2n complexes are the active intermediates in the electrolysis of CuII(CO3)n2−2n solution. The results enable the evaluation of E°[(CuIII/II(CO3)n)aq]≈1.42 V versus NHE at pH 8.4. This redox potential is in accord with the electrochemical report. As opposed to literature suggestions for water oxidation, the present results rule out single‐electron transfer from CuIII(CO3)n3−2n to yield hydroxyl radicals. Significant charge transfer from the coordinated carbonate to CuIII results in the formation of C2O62− by means of a second‐order reaction of CuIII(CO3)n3−2n. The results point out that carbonate stabilizes transition‐metal cations at high oxidation states, not only as a good sigma donor, but also as a non‐innocent ligand.