The synthesis of efficient water-oxidation catalysts demands insight into the only known, naturally occurring water-oxidation catalyst, the oxygen-evolving complex (OEC) of photosystem II (PSII). Understanding the water oxidation mechanism requires knowledge of where and when substrate water binds to the OEC. Mn catalase in its Mn(III) Mn(IV) state is a proteinmodel of the OEC's S-2 state. From O-17-labeled water exchanged into the di-mu-oxo di-Mn(III,IV) coordination sphere of Mn catalase, CW Q-band ENDOR spectroscopy revealed two distinctly different O-17 signals incorporated in distinctly different time regimes. First, a signal appearing after 2 h of O-17 exchange was detected with a 13.0 MHz hyperfine coupling. From similarity in the time scale of isotope incorporation and in the O-17 mu-oxo hyperfine coupling of the di-mu-oxo di-Mn(III,IV) bipyridine model (Usov, O. M.; Grigoryants, V. M.; Tagore, R.; Brudvig, G. W.; Scholes, C. P. J. Am. Chem. Soc. 2007, 129, 11886-11887), this signal was assigned to mu-oxo oxygen. EPR line broadening was obvious from this O-17 mu-oxo species. Earlier exchange proceeded on the minute or faster time scale into a non-mu-oxo position, from which O-17 ENDOR showed a smaller 3.8 MHz hyperfine coupling and possible quadrupole splittings, indicating a terminal water of Mn(III). Exchangeable proton/deuteron hyperfine couplings, consistent with terminal water ligation to Mn (III), also appeared. Q-band CW ENDOR from the S-2 state of the OEC was obtained following multihour O-17 exchange, which showed a O-17 hyperfine signal with a 11 MHz hyperfine coupling, tentatively assigned as mu-oxo-O-17 by resemblance to the mu-oxo signals from Mn catalase and the di-mu-oxo bipyridine model.