The aim of this study is to characterize the compounds grown on copper during the oxidation at low temperature (T < 523 K) by optical methods: photoluminescence and UV‐Visible‐NIR diffuse reflectance spectroscopy. Two cuprous oxides Cu₂O and Cu₃O₂ have been studied. The absorption of Cu₂O films in the range 450–630 nm is mainly due to non‐stoichiometry bands associated with copper and oxygen vacancies. Cu₃O₂ is characterized by an optical band gap greater than that of Cu₂O (respectively 2.25 and 1.95 eV) and by an intense luminescence emission at 760–780 nm. Cu₃O₂ may be considered as a gross defect structure of Cu₂O (a Cu 2O = 0.427 nm, a Cu 3O 2 = 0.431 nm). The experimental approach of the oxidation mechanism reveals that at 423 K Cu₂O is the primary product which later on is transformed into Cu₃O₂. Experimental and calculated optical absorption curves disclose the nucleation of CuO inside the cuprous oxides layer for oxidation in the range 473–523 K.