Aliphatic C-H bond oxygenation is achieved by Co(II)-peroxo species. The dinuclear Co(II)- mu-peroxo complex, {Co[HB(3,5-Pr(2)(i)pz)(3)]}(2)(mu-O-2) (2), is yielded by reaction of the bis(mu-hydroxo)-Co(II) complex, {Co(OH)[HB (3,5-Pr(2)(i)pz)(3)]}(2) (1), with an equimolar amount of H2O2. Spontaneous decomposition of the mu-peroxo complex 2 yields a mono-oxygenated mu-alkoxo-mu-hydroxo complex, CO2(mu-OH) [HB(mu-3-OCMe2-5-Pr(i)pz)(3,5-Pr(2)(i)pz)(2)] [HB(3,5-Pr(2)(i)pz)(3)] (4), In which one of the six 3-isopropyl methine carbon atoms is oxygenated and the resulting alkoxo ligand bridges the two Co(II) centers. In contrast, decomposition of 2 in the presence of an excess amount of H2O2 results in further oxygenation to give a mixture of the dinuclear Co(II)-bis(mu-alkoxo) complex, {Co[HB(mu-3-OCMe2-5-Pr(i)pz)(3,5-Pr(2)(i)pz)(2)]}(2) (5), and the mononuclear Co(II)-hydroxo alcohol complex, Co(OH)[HB (3-Me2COH-5-Pr(i)pz)(3-Me2COH-5-Pr(i)pz)(2)] (6). In the bis(mu-alkoxo) complex 5, one of the three 3-isopropyl groups in each ligand is functionalized to give the alkoxide group, which bridges the two metal centers. In the mononuclear hydroxo alcohol complex 6, all of the three 3-isopropyl groups are oxygenated. The Co(II) center is coordinated by the resulting alcohol ligand as well as an hydroxide. Reaction of 1 with 2 equiv of POOH (R = Bu-t and PhMe2C) at low temperature yields the blue thermally unstable alkylperoxo compound 3. The HB(3-Bu-t-5-Pr(i)pz)(3) derivative, Co(OOCMe2Ph) [HB (3-Bu-t-5-Pr(i)pz)(3)] (9), characterized successfully by X-ray crystallography contains the tetrahedral Co center. The monomeric alkylperoxo complexes 3 also decomposes to give the bis(mu-alkoxo) complex 5, but neither mono-oxygenated 4 nor fully oxygenated 6 is obtained. These observations suggest that the present aliphatic C-H bond oxygenations may proceed via homolysis of the O-O bond of the Co(II)-peroxo species.