A mononuclear side-on peroxocobalt(III) complex with a tetradentate macrocyclic ligand, [Co-III(TBDAP)(O-2)](+) (1), shows a novel and facile mode of dioxygenase-like reactivity with nitriles (R-C N; R = Me, Et, and Ph) to produce the corresponding mononuclear hydroximatocobalt(III) complexes, [CoIII(TBDAP)(R-C(=NO)O)](+), in which the nitrile moiety is oxidized by two oxygen atoms of the peroxo group. The overall reaction proceeds in one-pot under ambient conditions (ca. 1 h, 40 degrees C). O-18-Labeling experiments confirm that both oxygen atoms are derived from the peroxo ligand. The structures of all products, hydroximatocobalt(III) complexes, were confirmed by X-ray crystallography and various spectroscopic techniques. Kinetic studies including the Hammett analysis and isotope labeling experiments suggest that the mechanistic mode of 1 for activation of nitriles occurs-via a concerted mechanism. This novel reaction would be significantly valuable for expanding the chemistry for nitrile activation and utilization.