A titration method permitting the determination of the products of air oxidation of thin layers of cuprous sulfide containing copper or not is described. It consists of a cathodic reduction at constant current with potentiometric determination of the equivalent point (current density, 2 A m−2; electrolyte, 0.1M aqueous solution of sodium acetate). Nonstoichiometric cuprous sulfide is reduced to at −0.7V (vs. 1M electrode) and to copper at −1.0V. When cuprous sulfide is heated in air, cupric oxide is formed which is reduced at −0.16V. If an excess of metallic copper is present, cuprous oxide is formed and reduced at −0.45V. The titration method may be used to characterize the cuprous sulfide from solar cells.