Electronic absorption spectroscopy was used to study the complex formation in water-dioxane solutions containing Cu(II), 4,4'-bipyridyl (Bp), 1,10-phenanthroline (Phen), and ethylenediamine (En). The nonlinear least-squares method and factor analysis showed that the most satisfactory description of the experimental dependences of absorbances on the concentrations of solution components is provided by a mathematical model that includes, as parameters, stability constants (?) of homoligand and mixed-ligand ternary complexes and those of the quaternary [Cu2(Phen)(En)(Bp)]solv4+ complex. Based on the calculated electronic absorption spectra, the spectrochemical series for the compounds studied was constructed. The formation of the quaternary complex from mixed-ligand ternary complexes was found to result in a hypsochromic shift of the d-d absorption maximum.