ln-situ grazing incidence X-ray diffractometry (GIXD) was used to study the electrochemical oxidation and reduction of copper in solutions at pH 9, 12 and 14. Crystalline Cu2O and CuO films down to thicknesses of tens of nanometers were detected under such conditions, whereas a crystalline Cu(OH)(2) phase did not occur. The formation rate of crystalline Cu2O films increases with pH and formation time. In the Cu(II) potential region, Cu2O can coexist with amorphous Cu(OH)(2) phases. Crystalline CuO is not formed directly. At pH 14, it occurs by slow conversion of Na2CuO2 precipitates. Crystalline CuCl formation takes place only in the Cu(II) potential region at pH 9.