Understanding the mechanism for electrochemical water oxidation is important for the development of more efficient catalysts for artificial photosynthesis. A basic step is the proton-coupled electron transfer, which enables accumulation of oxidizing equivalents without buildup of a charge. We find that substituting deuterium for hydrogen resulted in an 87% decrease in the catalytic activity for water oxidation on Co-based amorphous-oxide catalysts at neutral pH, while 16O-to-18O substitution lead to a 10% decrease. In situ visible and quasi-in situ X-ray absorption spectroscopy reveal that the hydrogen-to-deuterium isotopic substitution induces an equilibrium isotope effect that shifts the oxidation potentials positively by approximately 60 mV for the proton coupled CoII/III and CoIII/IV electron transfer processes. Time-resolved spectroelectrochemical measurements indicate the absence of a kinetic isotope effect, implying that the precatalytic proton-coupled electron transfer happens through a stepwise mechanism in which electron transfer is rate-determining. An observed correlation between Co oxidation states and catalytic current for both isotopic conditions indicates that the applied potential has no direct effect on the catalytic rate, which instead depends exponentially on the average Co oxidation state. These combined results provide evidence that neither proton nor electron transfer is involved in the catalytic rate-determining step. We propose a mechanism with an active species composed by two adjacent CoIV atoms and a rate-determining step that involves oxygen-oxygen bond formation and compare it with models proposed in the literature.