A Co(II) complex, [Co(L)(2)]Cl-2, 1 of the ligand L (L = bis(2-ethyl-4-methylimidazol-5-yl)methane) upon reaction with H2O2 in methanol solution at -40 degrees C resulted in the formation of the corresponding Co(III)-peroxo complex [Co(L)(2)(O-2)]+ (2). The addition of NO gas to the freshly generated solution of the complex 2 led to the formation of the Co(II)-nitrato complex 3 through the putative formation of a Co(II)-peroxynitrite intermediate, 2a. The intermediate 2a was found to mediate the nitration of the externally added phenol resembling the nitration of tyrosine in biological systems.