The dimanganese(II,II) complexes 1a [Mn-2(L)(OAC)(2)(CH3OH)](ClO4) and 1b [Mn-2(L)(OBz)(2)(H2O)](ClO4), where HL is the unsymmetrical phenol ligand 2-(bis-(2-pyridylmethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminomethyl)-4-methylphenol, react with hydrogen peroxide in acetonitrile solution. The disproportionation reaction was monitored by electrospray ionization mass spectrometry (ESI-MS) and EPR and UV-visible spectroscopies. Extensive EPR studies have shown that a species (2) exhibiting a 16-line spectrum at g similar to 2 persists during catalysis. ESI-MS experiments conducted similarly during catalysis associate 2a with a peak at 729 (791 for 2b) corresponding to the formula [(MnMnIV)-Mn-III(L)(O)(2)(OAc)](+) ([(MnMnIV)-Mn-III(L)(O)(2)(OBz)](+) for 2b). At the end of the reaction, it is partly replaced by a species (3) possessing a broad unfeatured signal at g similar to 2. ESI-MS associates 3a with a peak at 713 (775 for 3b) corresponding to the formula [(MnMnIII)-Mn-II(L)(O)(OAc)](+) ([(MnMnIII)-Mn-II(L)(O)(OBz)](+) for 3b). In the presence of (H2O)-O-18, these two peaks move to 733 and to 715 indicating the presence of two and one oxo ligands, respectively, When (H2O2)-O-18 is used, 2a and 3a are labeled showing that the oxo ligands come from H2O2. Interestingly, when an equimolar mixture of H2O2 and (H2O2)-O-18 is used, only unlabeled and doubly labeled 2a/b are formed, showing that its two oxo ligands come from the same H2O2 molecule. All these experiments lead to attribute the formula [(MnMnII)-Mn-III(L)(0)2(OAc)]+ to 2a and to 3a the formula [(MnMnIII)-Mn-II(L)(O)(OAc)](+), Freeze-quench/EPR experiments revealed that 2a appears at 500 ms and that another species with a 6-line spectrum is formed transiently at ca. 100 ms. 2a was prepared by reaction of la with tert-butyl hydroperoxide as shown by EPR and UV-visible spectroscopies and ESI-MS experiments. Its structure was studied by X-ray absorption experiments which revealed the presence of two or three O atoms at 1.87 Angstrom and three or two N/O atoms at 2.14 Angstrom. In addition one N atom was found at a longer distance (2.3 Angstrom) and one Mn at 2.63 Angstrom. 2a can be one-electron oxidized at E-1/2 = 0.91 V-NHE (DeltaE(1/2) = 0.08 V) leading to its (MnMnIV)-Mn-IV analogue. The formation of 2a from la was monitored by UV-visible and X-ray absorption spectroscopies. Both concur to show that an intermediate (MnMnII)-Mn-II species, resembling 4a [Mn-2(L)(OAc)(2)(H2O)]-(ClO4)(2), the one-electron-oxidized form of la, is formed initially and transforms into 2a. The structures of the active intermediates 2 and 3 are discussed in light of their spectroscopic properties, and potential mechanisms are considered and discussed in the context of the biological reaction.