Polycrystalline copper electrodes were oxidised in 0.1 M KOHaq at −350 mV vs. Hg|HgO. The properties of such formed oxide layers were studied by means of X-ray photoelectron spectroscopy and impedance measurements. The layers are composed with Cu2O with properties of a p-type semiconductor. The impedance spectra recorded at potentials of Cu2O formation were found to meet the requirements of correct and valid impedance data. An analysis of data fitting errors allowed selection of the equivalent circuit optimal for Cu2O covered electrodes. The overall interfacial capacitance depends on the electrode potential and on the oxide thickness. Elements of the equivalent circuit describing capacitances of the double layer and of the oxide have been determined. It can be estimated that the double layer capacitance of the oxidised Cu electrode is comparable with the capacitance measured for the metallic surface at potentials of hydrogen evolution.