The copper atom reacts with dioxygen in solid argon to form a mononuclear copper dioxygen complex Cu(eta(2)- O-2)(eta(1)-O-2)(2) bearing both side-on and end-on bonded O-2 ligands. The complex is characterized as being a trisuperoxide species with the copper center in its unusual +3 oxidation state. The Cu(III) trisuperoxide complex loses an end-on bonded superoxo ligand in forming the copper bisdioxygen complex Under infrared irradiation.