Photosystem II (PSII) catalyzes the light driven oxidation of water and the reduction of plastoquinone. PSII is a multisubunit membrane protein; the D1 and D2 polypeptides form the heterodimeric core of the PSII complex. Water oxidation occurs at a manganese-containing oxygen evolving complex (OEC). PSII contains two redox active tyrosines, Y-Z and Y-D, which form the neutral tyrosyl radicals, Y-Z(center dot) and Y-D(center dot) Y-D has been assigned as tyrosine 160 in the D2 polypeptide through isotopic labeling and site-directed mutagenesis. Whereas YD is not directly involved in the oxidation of water, it has been implicated in the formation and stabilization of the OEC. PSII structures have shown Y-D to be within hydrogen-bonding distance of histidine 189 in the D2 polypeptide. Spectroscopic studies have suggested that a proton is transferred between Y-D and histidine 189 when Y-D is oxidized and reduced. In our previous work, we used (H2O)-H-2 solvent exchange to demonstrate that the mechanism of Y-D proton-coupled electron transfer (PCET) differs at high and low pH. In this article, we utilize the proton inventory technique to obtain more information concerning PCET mechanism at high pH. The hypercurvature of the proton inventory data provides evidence for the existence of multiple, proton-donation pathways to Y-D(center dot). In addition, at least one of these pathways must involve the transfer of more than one proton.