Complex-formation equilibria of some dipeptides (glycyl-lysine, alanyl-lysine, histidyl-lysine and beta -alanyl-lysine) with the Cu(II) ion have been studied in aqueous solution, at I = 0.1 mol dm(-3) (KNO3) and T = 25 degreesC. Protonation and complex-formation constants have been determined potentiometrically; complex-formation model and species stoichiometry have been checked carefully by means of different spectroscopic techniques (visible absorption, CD, EPR). The structure of the main complex species is discussed, on the basis of thermodynamic data obtained by direct calorimetry. The formation of binuclear species is demonstrated and it is suggested that the side amino group of lysine participates in complex formation at alkaline pH values. (C) 2000 Elsevier Science B.V. All rights reserved.