[Mn-2(III/IV)(mu-O)(2)(terpy)(2)(OH2)(2)](NO3)(3) (1, where terpy = 2,2':6'2' '-terpyridine) + oxone (2KHSO(5)center dot KHSO4 center dot K2SO4) provides a functional model system for the oxygen-evolving complex of photosystem II that is based on a structurally relevant Mn-(mu-O)(2)-Mn moiety (Limburg, J.; et al. J. Am. Chem. Soc. 2001, 123, 423-430). In this study, electron paramagnetic resonance, ultraviolet-visible spectroscopy, electrospray ionization mass spectrometry, X-ray absorption spectroscopy, and gas-phase stable isotope ratio mass spectrometry were utilized to identify the title compounds in the catalytic solution. We find that (a) O-2 evolution does not proceed through heterogeneous catalysis by MnO2 or other decomposition products, that (b) O atoms from solvent water are incorporated into the evolved O-2 to a significant extent but not into oxone, that (c) the Mn-2(III/IV) title compound 1 is an active precatalyst in the catalytic cycle of O-2 evolution with oxone, while the Mn-2(IV/IV) oxidation state is not, and that (d) the isotope label incorporation in the evolved O-2, together with points a-c above, is consistent with a mechanism involving competing reactions of oxone and water with a "(MnO)-O-V" intermediate in the O-O bond-forming step.