The complex [(terpy)(H2O)Mn-III(O)(2)Mn-IV(OH2)(terpy)](NO3)(3) (terpy = 2,2':6,2"-terpyridine) (1) catalyzes O-2 evolution from either KHSO5 (potassium oxone) or NaOCl. The reactions follow Michaelis-Menten kinetics where V-max = 2420 +/- 490 mol O-2 (mol 1)(-1) hr(-1) and K-M = 53 +/- 5 mM for oxone ([1] = 7.5 muM), and V-max = 6.5 +/- 0.3 mol O-2 (mol 1)(-1) hr(-1) and K-M = 39 +/- 4 mM for hypochlorite ([1] = 70 muM), with first-order kinetics observed in 1 for both oxidants. A mechanism is proposed having a preequilibrium between 1 and HSO5- or OCl-, supported by the isolation and structural characterization of [(terpy)(SO4)Mn-IV(O)(2)Mn-IV(O4S)(terpy)] (2). Isotope-labeling studies using (H2O)-O-18 and (KHSO5)-O-16 show that O-2 evolution proceeds via an intermediate that can exchange with water, where Raman spectroscopy has been used to confirm that the active oxygen of HSO5- is nonexchanging (t(1/2) >> 1 h). The amount of label incorporated into O-2 is dependent on the relative concentrations of oxone and 1. O-32(2):O-34(2):O-96(2) is 91.9 +/- 0.3:7.6 +/- 0.3:0.51 +/-0.48, when [HSO5-] = 50 mM (0.5 mM 1), and 49 +/- 21:39 +/-15:12 +/- 6 when [HSO5-] = 15 mM (0.75 mM 1). The rate-limiting step of Oz evolution is proposed to be formation of a formally Mn-V=O moiety which could then competitively react with either oxone or water/hydroxide to produce O-2. These results show that 1 serves as a functional model for photosynthetic water oxidation.