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Plants and cyanobacteria produce atmospheric dioxygen from water, powered by sunlight and catalyzed by a manganese complex in photosystem II. A classic S-cycle model for oxygen evolution involves five states, but only four have been identified. The missing S₄ state is particularly important because it is directly involved in dioxygen formation. Now progress comes from an x-ray technique that can monitor redox and structural changes in metal centers in real time with 10-microsecond resolution. We show that in the O₂-formation step, an intermediate is formed—the enigmatic S₄ state. Its creation is identified with a deprotonation process rather than the expected electron-transfer mechanism. Subsequent electron transfer would give an additional S₄′ state, thus extending the fundamental S-state cycle of dioxygen formation.