to Co(IV)-O -, here illustrated as a 2e -, 2H + process. However, the presence of Co(IV) is not enough to achieve highly efficient OER catalysis as Co(IV) was observed even at pH 11, in the absence of significant catalysis. Instead, there is another deprotonation step, likely that of a bridging oxygen atom, either between Co and Co or Co and Fe. This deprotonation, along with the 2e -, 2H + Co(III/IV) transition give rise to the observed 102 mV/pH, 2e -, 3H + , rate dependence of the OER of Co0.5Fe0.5O0.5F1.5 catalysts. The rapid drop in Tafel slope until pH 12 indicates that the pKa of this bridging oxygen at 1.4VRHE is approximately 12.