The solar water-splitting protein complex, photosystem II catalyzes one of the most energetically demanding reactions in nature by using light energy to drive water oxidation. The four-electron water oxidation reaction occurs at the tetranuclear manganese-calcium-oxo cluster that is present in the oxygen-evolving complex of photosystem II. The tetranuclear manganese-calcium-oxo cluster is comprised of mixed-valence Mn(III) and Mn(IV) ions in the ground state. The oxo-manganese dimer, [H2O(terpy)Mn-III(mu-O)(2)Mn-IV(terpy)OH2](NO3)(3) (terpy = 2,2':6',2 ''-terpyridine) (1), is an excellent biomimetic model that has been extensively used to gain insight on the molecular structure and mechanism of water oxidation in photosystem H. In this work, weak magnetic interactions between the protons of the two terminal water ligands and the paramagnetic dimanganese "di-mu-oxo" core of I are quantitatively characterized using two-dimensional hyperfine sublevel correlation (HYSCORE) spectroscopy. For the water molecule that is directly coordinated at the Mn(III) ion, the two protons are found to be magnetically equivalent and exhibit near axial hyperfine anisotropy. In contrast, for the first time, we demonstrate that the two protons of the water molecule that is directly coordinated at the Mn (IV) ion are inequivalent. We obtain the isotropic and anisotropic components of the hyperfine interaction for each proton. A comparison of the HYSCORE spectra measured in the presence and absence of acetate ions provides unambiguous evidence that only one molecule of acetate binds to 1 by replacing a terminal water molecule that is coordinated at the Mn(III) ion.