The oxygen-evolving complex of photosystem II can function with either Ca(2+) or Sr(2+) as the heterocation, but the reason for different turnover rates remains unresolved despite reported X-ray crystal structures for both forms. Using quantum mechanics/molecular mechanics (QM/MM) calculations, we optimize structures with each cation in both the resting state (S1) and in a series of reduced states (S0, S-1, and S-2). Through comparison with experimental data, we determine that the X-ray crystal structures with either Ca(2+) or Sr(2+) are most consistent with the S-2 state (i.e., Mn4[III,III,III,II] with O4 and O5 protonated). As expected, the QM/MM models show that Ca(2+)/Sr(2+) substitution results in the elongation of the heterocation bonds and the displacement of terminal waters W3 and W4. The optimized structures also show that hydrogen-bonded W5 is displaced in all S states with Sr(2+) as the heterocation, suggesting that this water may play a critical role during water oxidation.