A novel dicobalt(II) mu-peroxo complex (3) is formed by the reaction of a dicobalt(II) bis(mu-hydroxo) complex, [Co(HB(3,5-iPr(2)pz)(3))](2)(OH)(2) (2), with a stoichiometric amount of H2O2. Unusual and characteristic spectroscopic properties [(1) existence of two O-2(2-) to metal charge transfer absorption bands; (2) low v(O-O) vibration in the resonance Raman spectrum; (3) antiferromagnetic coupling between the two Co(II) ions] suggest that the peroxo ligand coordinates in the planar mu-eta(2):eta(2) binding mode as found in the oxyhemocyanin and the dicopper(II) peroxo model complexes. Warming of a pentane solution of 3 induces aliphatic C-H bond activation concomitant with O-O bond cleavage to yield the novel mu-alkoxo-mu-hydroxo complex 4, Co-2(mu-OH)[HB(mu-3-OCMe(2)-5-iPrpz)(3,5-iPr(2)-pz)(2)][HB(3,5-iPr(2)pz)(3)], and the bis(mu-hydroxo) complex 2.