Cobalt(II) salts react with H2O2 in the presence of 2 equiv of tetramethylethylenediamine (tmen) to produce a stable sideways bonded mononuclear peroxo complex [Co(tmen)(2)O-2]ClO4 which has been characterized chemically and by a single crystal X-ray determination. This is the first such tetraaminecobalt(III) complex, indeed, the first for any tetraaminemetal ion complex. The mononuclear peroxo complex can also be synthesized by O-2(2-)-anation of cis-[Co(tmen)(2)(OH2)(2)](3+). This reaction is reversed in acid, and this offers the potential to develop Co(III) catalyzed oxidation reactions.