An attempt has been made to account for the differences in the values of the observed diffusion coefficient of Cu-II at a transition from a separate reduction to a codischarge with Ni and Mo. For this purpose experiments were carried out in a wide range of pH for various electrolyte compositions, combining them with the analysis of species occurring in these solutions and with the analysis of different models of mass transport to the cathode surface. The observed results indicate that in the range pH = 6-8, the changes in the values of the limiting currents of Cu occurring in the presence of nickel ions are associated with the formation of a inert heteronuclear complex [CuNi(cit)(2)](4-). The obtained results also help to account for the differences in the value of the diffusion coefficient of Cu-II obtained in such electrolytes by other researchers. They also form a basis for the preparation of optimum bath compositions and for further explanation of the electrodeposition mechanism of Ni-Cu and Ni-Cu-Mo alloys.