The mechanism of synergetic adsorptive accumulation of copper(II)–phenanthroline–tributylphosphate (Cu–phen–TBP) complex at the mercury drop electrode surface has been described. The behaviour and characteristics of the CuII mixed ligand complex was studied and the relationship between the properties of square-wave response and the parameters of a charge transfer and of the excitation signal are discussed. Square-wave voltammetric response of the Cu-phen-TBP complex are analysed according to the theoretical results.