The dioxo-bridged MnIIIMnIV complexes [MnIIIMnIV(μ-O)2L4]3+(13+, L=bipy=2,2′-bipyridine; 23+, L=phen=1,10-phenanthroline) can be stabilized in aqueous solutions containing excess L in the rangepH 4–6. Thus stabilized in solution, the parent complex coexists only with its aqua derivative [MnIIIMnIV(μ-O)2L3(H2O)2]3+ (1a3+ for L=bipy;2a3+ for L=phen) in rapid equilibrium. The solutions are novel oxidants and quantitatively oxidize NO2− to NO3−, S2O32− to S4O62−, H2O2 to O2 and N2H5+ to N2. Simple first-order kinetics is observed except in reactions of the bipy complex with NO2−, H2O2 and in the reaction of the phen complex with N2H5+. Reaction of the bipy complex with NO2− produces the one-electron reduced intermediate1a2+, while H2O2 gives the two-electron reduced intermediate [MnIII(bipy)2(OH)(H2O)]2+. It is concluded that (a) the aqua complexes are kinetically more active than their parents, (b) the phen complexes react slower than the bipy-complexes, (c) Mn2+ has no catalytic role, but intermediate manganese complexes and radical species may act as autocatalysts, (d) outer-sphere one-electron transfer occurs generally, but H2O2 prefers a two-electron pathway in its reactions with the dimers, (e) nuclearity is retained until one of the manganese in the dimers is reduced to the +2 state.