Chronopotentiometry (CP) using 0.1 M KCl as the electrolyte has been most frequently used for selective determination of cuprous and cupric oxides (Cu2O and CuO) formed on copper surfaces. However, there are conflicting views regarding the order of reduction of the oxides. This study was carried out to settle this problem. Differently prepared samples of Cu-duplex oxide films were partially reduced by means of CP with 0.1 M KCl and then submitted to X-ray diffractometry and also linear sweep voltammetry using a strongly alkaline electrolyte (6 M KOH + 1 M LiOH) in which reduction peaks of Cu2O and CuO could be obtained with good separation. The results clearly demonstrated that CuO was first reduced in 0.1 M KCl, followed by the reduction of Cu2O. However, the reductions of both oxides were found to occur simultaneously, to a greater or lesser extent, in 0.1 M KCl. It was also revealed that CuO was reduced to metal Cu in one step and that a partially reduced Cu-duplex oxide sample had an expected Cu vertical bar Cu2O vertical bar Cu sandwich structure. (c) 2006 The Electrochemical Society.