The course of the conversion reaction during the electrochemical insertion/deinsertion of lithium in CuO thin film electrodes was surveyed by cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS). The electrochemical processes were studied through the comprehensive acquisition and interpretation of XPS and Auger spectra (Cu 2p, O 1s, Cu LMM) recorded at different stages of the first cycle of a Li/CuO cell. The reduction process consisting of three main steps leads successively to Cu+ and Cu0 and involves a Li2O2 peroxide type phase as an intermediate, whereas Li2O is the main lithiated oxide at the end of the discharge. Only the two last steps appear reversible in charge. No evidence of Cu2+ was found at the end of the charge at 3.5 V/Li+/Li, showing the irreversibility of the first step in our experimental conditions. Complementary XPS depth profile analyses were performed to check the evolution of the active material composition over the thickness of the electrode.