In photosystem II (PSII), water oxidation occurs at a Mn4CaO5 cluster and results in production of molecular oxygen. The Mn4CaO5 cluster cycles among five oxidation states, called S-n states. As a result, protons are released at the metal cluster and transferred through a 35 A hydrogen-bonding network to the lumen. At 283 K, an infrared band at 2830 cm(-1) is assigned to an internal solvated hydronium ion via (H2O)-O-18 solvent exchange. This result is similar to a previous report at 263 K Computations on an oxygen evolving complex model predict that chloride can stabilize a hydronium ion on a network of nine water molecules. In this model, a H3O+ stretching mode at 2738 CM-1 is predicted to shift to higher frequency with bromide and to lower frequency with nitrate substitution. The calculated frequencies were compared to S-2-minus-S-1 reaction-induced Fourier transform infrared spectra acquired from chloride-, bromide-, or nitrate-containing PSII samples, which were active in oxygen evolution. As predicted, the frequency of the 2830 cm(-1) band shifted to higher energy with bromide and to lower energy with nitrate substitution. These results support the conclusion that an internal hydronium ion and chloride play a direct role in an internal proton transfer event during the 51-to-S2 transition.