The reactivity of mononuclear metal-hydroperoxo adducts has fascinated researchers in many areas due to their diverse biological and catalytic processes. In this study, a mononuclear cobalt(III)-peroxo complex bearing a tetradentate macrocyclic ligand, [Co-III(Me-3-TPADP)(O-2)](+) (Me-3-TPADP = 3,6,9- trimethy1-3,6,9-triaza-1(2,6)-pyridinacydodecaphane), was prepared by reacting [Co-II(Me-3-TPADP)(CH3CN)(2)](2+) with H2O2 in the presence of triethylamine. Upon protonation, the cobalt(III)-peroxo intermediate was converted into a cobalt (III)-hydroperoxo complex, [Co-III(Me-3-TPADP) (O2H) (CH3CN)](2+). The mononuclear cobalt(III)-peroxo and -hydroperoxo intermediates were characterized by a variety of physicochemical methods. Results of electrospray ionization mass spectrometry clearly show the transformation of the intermediates: the peak at m/z 339.2 assignable to the cobalt(III)-peroxo species disappears with concomitant growth of the peak at m/z 190.7 corresponding to the cobalt(III)-hydroperoxo complex (with bound CH3CN). Isotope labeling experiments further support the existence of the cobalt(III)-peroxo and hydroperoxo complexes. In particular, the O-O bond stretching frequency of the cobalt(III)-hydroperoxo complex was determined to be 851 cm(-1) for (O2H)-O-16 samples (803 cm(-1) for (O2H)-O-18 samples), and its Co-O vibrational energy was observed at 571 cm(-1) for (O2H)-O-16 samples (551 cm(-1) for (O2H)-O-18 samples; 568 cm(-1) for (O2H)-O-16-H-2 samples) by resonance Raman spectroscopy. Reactivity studies performed with the cobalt(III)-peroxo and hydroperoxo complexes in organic functionalizations reveal that the latter is capable of conducting oxygen atom transfer with an electrophilic character, whereas the former exhibits no oxygen atom transfer reactivity under the same reaction conditions. Alternatively, the cobalt(III)-hydroperoxo complex does not perform hydrogen atom transfer reactions, while analogous low-spin Fe(III)- hydroperoxo complexes are capable of this reactivity. Density functional theory calculations indicate that this lack of reactivity is due to the high free energy cost of O-O bond homolysis that would be required to produce the hypothetical Co(IV)-oxo product.