Photosystem II oxidizes water at a Mn4CaO5 cluster. Oxygen evolution is accompanied by proton release through a 35 #x212B; hydrogen-bonding network to the lumen. The mechanism of this proton-transfer reaction is not known, but the reaction is dependent on chloride. Here, vibrational spectroscopy defines the functional properties of the proton-transfer network using chloride, bromide, and nitrate as perturbative agents. As assessed by peptide C=O frequencies, bromide substitution yields a spectral Stark shift because of its increase in ionic radius. Nitrate substitution leads to more complex spectral changes, consistent with an overall increase in hydrogen-bonding interactions with the peptide backbone. The effects are similar to spectral changes previously documented in site-directed mutations in a putative lumenal pathway. Importantly, the effects of nitrate are reversed by the osmolyte, trehalose. Trehalose is known to alter hydrogen-bonding interactions in proteins. Trehalose addition also reverses a shift in an internal hydronium ion signal, consistent with an alteration in its pK(a) value and a change in the basicity of bound nitrate. The spectra provide evidence that the proton-transfer pathway contains peptide carbonyl groups, internal water, a hydronium ion, and amino acid side chains. These experiments also show that the proton-transfer pathway functionally adapts to changes in electric field, pK(a), and hydrogen bonding and thereby optimizes proton transfer to the lumen.