Oxide layers thermally formed on copper have been studied using double sweep cyclic voltammetry in strongly alkaline electrolytes. It was found that the addition of 1 M LiOH in an electrolyte (6 M KOH) allowed perfect resolution of cathodic waves due to the reduction of Cu2O and CuO. Assignment of the two reduction waves has been achieved with the help of spectrophotometric techniques including X-ray photoelectron spectroscopy and X-ray diffraction; the cathodic wave appearing between -1.3 and -1.5 V (vs. Ag/AgCl) was attributed to the reduction of Cu2O, while that appearing at a less negative potential (-1.0 to -1.1 V) was attributed to the reduction of CuO. The electrochemical measurement of samples prepared under several conditions has revealed that CuO is reduced at once to Cu prior to the reduction of Cu2O. It was also confirmed that the formation of the oxide films was accelerated by elevating temperature, heightening humidity, and by preimmersion in electrolyte solutions. Water vapor was essential for the formation of CuO at a lower temperature (80 degreesC).