The nature of the Mn oxidation states involved in photosynthetic oxygen evolution has remained controversial, despite intense study by X-ray absorption and electron paramagnetic resonance spectroscopy. As on alternative approach, high-resolution K beta X-ray fluorescence spectra have been recorded on the dark-adapted S-1 state and the hydroquinone-reduced state of the oxygen-evolving complex in photosystem II. By comparison of the K beta chemical shifts with those of appropriate model compounds, the S-1 state of photosystem II is found to contain equal amounts of Mn(III) and Mn(IV). In the hydroquinone-reduced sample, a significant fraction of the Mn is reduced to Mn(II). The results are compatible with models involving conversion of Mn(III)(2)Mn(IV)(2) to Mn(II)(2)Mn(IV)(2) clusters.