The complexes [Mn(2-OH(X-sal)pn)]2(n-) (where X = 5-OCH3, H, 5-Cl, 3,5-diCl, or 5-NO2 and where n = 0 or 2) are shown to be excellent hydrogen peroxide disproportionation catalysts in acetonitrile. When : carried out in an open vessel, the reaction can occur for-over 5000 turnovers without an indication of catalyst decomposition. The disproportionation reaction cycles between the [Mn-III(2-OH(X-sal)pn)](2) and the [Mn-II(2-OH(X-sal)pn)](2)(2-) oxidation levels. All derivatives show saturation kinetics,with the highest k(cat) (21.9 +/- 0.2 s(-1)) observed for the [Mn-III(2-OH(5-Clsal)pn)](2) dimer and the optimal k(cat)/K-M (990 +/- 69 s(-1.)M(-1)) observed for the [Mn-III(2-OHsalpn)](2). The first step of the reaction is proposed to be the binding of peroxide to the [Mn-III(2-OH(X-sal)pn)](2) through an alkoxide shift to form a ternary intermediate {[Mn-III(2-OH(X-sal)pn)](2)(H2O2)}. We propose that the turnover-limiting step is the oxidation of peroxide from this intermediate. The binding efficiency of the peroxide is dependent on the phenyl-ring substitution with the derivatives donating the most electrons having the highest affinity for the substrate. Studies with isotopically labeled H2O2 indicate that protons are important-in the turnover-limiting step of the reaction and that the O-O bond is not cleaved during peroxide oxidation. In a closed vessel, the product dioxygen will oxidize [Mn-II(2-OH(5-NO(2)sal)pn)](2)(2-) to [Mn-II/III(2-OH(5-NO(2)sal)pn)(2)](-), and this species can then be stoichiometrically oxidized by hydrogen peroxide to give [Mn-III/IV(2-OH(5-NO(2)sal)pn)(2)(mu(2)-O)(2)](-). This di-mu(2)-oxobridged species is catalytically incompetent; however, addition of hydroxylamine hydrochloride restores catalytic activity. The relationship of this catalytic disproportionation of hydrogen peroxide and inactivation of the catalyst will be used to define a model for similar reactions' observed for the Lactobacillus plantarum Mn catalase.