The influence of film thickness on the formation of cupric oxide in cuprous oxide films on an abraded copper surface was studied from measurements obtained by the electrolytic reduction method. The critical thickness value for the formation of cupric oxide in the films appeared to be near 600A for a commercial copper, and the critical thickness range was found to be approximately 400 to 800A. The thicknesses resulting from heating specimens in different atmospheres, for a given time, increase with the oxygen concentration. The time required to produce films of a given thickness decreases as the oxygen concentration increases. Murison's observations concerning the conditions favorable to the formation of cupric oxide in the films at a higher temperature were extended to include the proper period of heating and film thickness as well as oxygen concentration. Dunholter and Kersten's electron diffraction analyses of copper oxide films on mirror‐like surfaces were considered to be in satisfactory agreement with the present results.