The O2 and NO reactivity of a Cr(II) complex bearing a 12-membered tetraazamacrocyclic N-tetramethylated cyclam (TMC) ligand, [Cr II(12-TMC)(Cl)]+ (1), and the NO reactivity of its peroxo derivative, [CrIV(12-TMC)(O2)(Cl)]+ (2), are described. By contrast to the previously reported Cr(III)-superoxo complex, [CrIII(14-TMC)(O2)(Cl)]+, the Cr(IV)-peroxo complex 2 is formed in the reaction of 1 and O2. Full spectroscopic and X-ray analysis revealed that 2 possesses side-on ν2-peroxo ligation. The quantitative reaction of 2 with NO affords a reduction in Cr oxidation state, producing a Cr(III)-nitrato complex, [CrIII(12-TMC) (NO3)(Cl)]+ (3). The latter is suggested to form via a Cr(III)-peroxynitrite intermediate. [CrII(12-TMC)(NO)(Cl)] + (4), a Cr(II)-nitrosyl complex derived from 1 and NO, could also be synthesized; however, it does not react with O2.