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Structural and electronic changes (oxidation states) of the Mn₄Ca complex of photosystem II (PSII) in the water oxidation cycle are of prime interest. For all four transitions between semistable S-states (S₀ → S₁, S₁ → S₂, S₂ → S₃, and S₃,₄ → S₀), oxidation state and structural changes of the Mn complex were investigated by X-ray absorption spectroscopy (XAS) not only at 20 K but also at room temperature (RT) where water oxidation is functional. Three distinct experimental approaches were used:  (1) illumination-freeze approach (XAS at 20 K), (2) flash-and-rapid-scan approach (RT), and (3) a novel time scan/sampling-XAS method (RT) facilitating particularly direct monitoring of the spectral changes in the S-state cycle. The rate of X-ray photoreduction was quantitatively assessed, and it was thus verified that the Mn ions remained in their initial oxidation state throughout the data collection period (>90%, at 20 K …