Cathodic stripping is used to determine the oxide thickness on copper. For copper oxidized at elevated temperatures the potential-time curve at constant cathodic current density features two distinct regions, corresponding to the reduction of two different oxides. The sequence in which the oxides are reduced at constant current density has been examined here using potential-time measurements and electron spectroscopy for chemical analysis. The results show that the top layer of cupric oxide, if continuous, is cathodically reduced first at a higher potential, followed by reduction of the inner layer of cuprous oxide at a lower potential. This conclusion is at variance with a reduction model used in measurements of the oxide thickness in the industry and accepted as part of a standard specification.