Electrochemical Impedance Spectroscopy (EIS) was applied on Cu-Ni samples passivated in a specially designed flat electrochemical cell which was heated at 60 degrees C. The electrolyte consisted of 160 g l(-1)H(2)SO(4), 40 g l(-1) Cu2+ and 0, 10, 20, 30 or 40 g l(-1) Ni2+ and the copper anodes contained nickel ranging from 0 w% to 10 w%. The oxygen content of the anodes and the electrolyte was also measured. An AC excitation signal of 10 mV and of 1mHz 100 MHz frequency was applied at the open circuit potential as well as at a passivation potential, the latter having been determined previously. The results indicate that nickel ion additions to the electrolyte increased the resistance of the electrolyte and altered the porosity, thickness and constituents of the passivation layer formed. The equivalent circuit models generated from the data acquired during the EIS experiments and the values for the electrical components were in the predicted range. The results are supported by supplementary XRD and SEM findings.