It has been shown that under comparable conditions the use of the Haring cell and the mercury reference electrode leads to similar values of the cathode polarization potential. These two methods have been used to study the variation with current density and with concentration of the total cathode polarization, concentration polarization potential, and activation overpotential of copper cathodes in copper sulfate‐sulfuric acid solutions prepared by the method developed earlier (1). Even under rigidly controlled conditions of preparation there appears to be a certain instability of solutions containing high concentrations of copper sulfate, as is manifested by variations in the polarization potential on repeated electrolysis of the solution. The variation of the polarization potentials with current density is discussed in relation to theory.