A 13-membered ring cyclic tetrapeptide was synthesized by the solid-phase peptide synthesis method, and its copper(II) coordination properties were analyzed by optical spectroscopy, mass spectrometry, and electrochemistry. All collected data strongly support the presence, at alkaline pH, of a stable peptide/copper(III) complex that is formed in solution by atmospheric dioxygen oxidation. On the basis of previous studies on cyclic peptide/copper systems, we suggest that the copper(III) ion is at the center of the ligand's cavity being coordinated to four deprotonated amide nitrogen atoms. This donor set would greatly lower the redox potential for the CuIII/CuII couple, thus allowing easy oxidation of the coordinated copper(II) by atmospheric oxygen.