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⁵⁵Mn pulse ENDOR experiments at 34 GHz (Q-band) are reported for the S₀ and S₂ states of the oxygen-evolving complex of photosystem II. Their numerical analysis (i) shows that in both states all four Mn ions are magnetically coupled, (ii) allows a refinement of the hyperfine interaction (HFI) parameters obtained earlier for the S₂ state at X-band (Peloquin, J. M.; Campbell, K. A.; Randall, D. W.; Evanchik, M. A.; Pecoraro, V. L.; Armstrong, W. H.; Britt, R. D. J. Am. Chem. Soc. 2000, 122, 10926−10942), (iii) provides the first reliable ⁵⁵Mn HFI tensors for the S₀ state, and (iv) leads to the suggestion that the Mn oxidation states in S₀ and S₂ are Mn₄(III, III, III, IV) and Mn₄(III, IV, IV, IV), respectively. In addition, a Q-band EPR spectrum is reported for the S₀ state, and inversion−recovery experiments at 4.5 K directly show that the electron spin−lattice relaxation for the S₀ state is about 2 orders of magnitude faster than that …