Copper(II) ion forms mixed complexes, CuAB, containing two different ligand molecules, with each of the six possible pairs of the ligands ethylenediamine, histamine, serine monoanion, and salicylate dianion. A general computer programme has been developed to calculate the stability constants of these mixed complexes from pH titration data on 1 : 1 : 1 mixtures of copper(II) and the two ligands. These constants are of comparable magnitude with the constants for the corresponding bis-complexes CuA2 and CuB2.