The O-2-evolving complex (OEC) of photosystem II (PSII) catalyzes the oxidation of water to dioxygen. In addition to a tetramanganese-oxo (Mn-4) Cluster, calcium and chloride ions, the OEC also contains Tyrosine Z(Y-Z), a redox intermediate in the water oxidation reaction. The redox mechanism employed by Y-Z is under much debate. Using a novel method to study Y-Z oxidation based on the kinetic competition with secondary donors, we examine the electron-donation pathways of manganese-depleted PSII over a range of temperature and pH. H/D substitution causes a shift in the onset temperature for Y-Z oxidation, enabling measurements of lyonium. isotope effects. In deuterated samples, the onset temperature for Y-Z oxidation is upshifted, suggesting that proton movement is a required step. Proton inventory experiments were performed to determine the number of protons that shift during the Y-Z oxidation reaction. Our findings indicate the movement of a single proton during the rate-limiting step of the oxidation process. The results presented herein demonstrate a need for proton movement in conjunction with Y-Z oxidation and support previous proposals that a proton-coupled electron transfer (PCET) step is necessary for oxidation of Y-Z. The possible involvement of PCET in the energetics of specific steps in the mechanism of water oxidation is discussed.