Stoichiometry, stability constants and solution structures of the copper(II) complexes of the N-acetylated tetrapeptide HisGlyHisGly were determined in aqueous solution in the pH range 2-11. The potentiometric and spectroscopic data (UV-Vis, CD, EPR and Raman scattering) show that acetylation of the amino terminal group induces drastic changes in the coordination properties of AcHGHG compared to HGHG. The N3 atoms of the histidine side chains are the first anchoring sites of the copper(II) ion. At pH 4.7 and 5.6 both the imidazole rings cooperate in the formation of a 2N equatorial set, while, at higher pH values, 3N and 4N complexes are formed through the coordination of peptide N- atoms. The logbeta values of the copper complexes of AcHGHG are by far lower than those of the corresponding species in the parent CuII-HGHG system.