Although there have been reports describing the nucleophilic reactivity of peroxomanganese(III) intermediates, as well as their conversion to high-valent oxo-bridged dimers, it remains a challenge to activate peroxomanganese(III) species for conversion to high-valent, mononuclear manganese complexes. Herein, we report the generation, characterization, and activation of a peroxomanganese(III) adduct supported by the cross-clamped, macro cyclic Me2EBC ligand (4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). This ligand is known to support high-valent, mononuclear Mn-IV species with well-defined spectroscopic properties, which provides an opportunity to identify mononuclear Mn-Iv products from O-O bond activation of the corresponding Mn-III-peroxo adduct. The peroxomanganese(III) intermediate, [Mn-III(O-2)(Me2EBC)(+), was prepared at low-temperature by the addition of KO2 to [Mn-II(Cl)(2)(Me2EBC)] in CH2C12, and this complex was characterized by electronic absorption, electron paramagnetic resonance (EPR), and Mn K-edge X-ray absorption (XAS) spectroscopies. The electronic structure of the [Mn-III(O-2)(Me2EBC)](+) intermediate was examined by density functional theory (DFT) and time-dependent (TD) DFT calculations. Detailed spectroscopic investigations of the decay products of [Mn-III(O-2)(Me2EBC)](+) revealed the presence of mononuclear Mn-III-hydroxo species or a mixture of mononuclear Mn-IV and Mn-III-hydroxo species. The nature of the observed decay products depended on the amount of KO2 used to generate Mn-III(O-2)(Me2EBC)](+). The Mn-III-hydroxo product was characterized by Mn K-edge XAS, and shifts in the pre-edge transition energies and intensities relative to [Mn-III(O-2)(Me2EBC)](+) provide a marker for differences in covalency between peroxo and nonperoxo ligands. To the best of our knowledge, this work represents the first observation of a mononuclear Mn-IV center upon decay of a nonporphyrinoid Mn-III-peroxo center.