The binding and activation of dioxygen by transition metal complexes is a fundamentally and partially important process in chemistry. Often the initial steps involve formation of peroxometal species that is difficult to observe because of their inherent reactivity. The interaction of dioxygen with a manganese(II) complex (1) of bis[(N'-tert-butylurealy)-N-ethyl]-(6-pivalomido-2-pyridylmethyl)amine was investigated, leading to the detection of a new intermediate that is peroxomanganese(III) complex (2). This complex is high-spin (S = 2) with a g value of 8.2 and D = - 2.0(5) as determined by parallel-mode electron paramagnetic resonance spectroscopy. The coordination of a peroxo ligand was established using Fourier transform infrared spectroscopy that reveals a new signal at 885cm(-1) for 2 when formed from O-16(2)-this band shifts to 837cm(-1) when O-18(2) is used in the preparation. Moreover , electrospray ionization mass spectra contain a strong ion at an m/z of 576.2703 for the O-16-isotopomer that shifts to 580.2794 in the O-18-isotopomer. Complex 2 also is capable of oxidatively deformylating aldehydes, which is a known reaction of peroxometal complexes. The similarities of 2 to the peroxo intermeidates in cytochrome P450 are noted.