CuFeO 2 , the structure prototype of the delafossite family, has received renewed interest in recent years. Thermodynamic modeling and several experimental Cu−Fe−O system investigations did not focus specifically on the possible nonstoichiometry of this compound, which is, nevertheless, a very important optimization factor for its physicochemical properties. In this work, through a complete set of analytical and thermostructural techniques from 50 to 1100°C, a fine reinvestigation of some specific regions of the Cu−Fe−O phase diagram under air was carried out to clarify discrepancies concerning the delafossite CuFeO 2 stability region as well as the eutectic composition and temperature for the reaction L = CuFeO 2 + Cu 2 O. Differential thermal analysis and Tammann's triangle method were used to measure the liquidus temperature at 1050 ± 2°C with a eutectic composition at Fe/(Cu + Fe) = 0.105 mol %. The quantification of all of the present phases during heating and cooling using Rietveld refinement of the high-temperature X-ray diffraction patterns coupled with thermogravimetric and differential thermal analyses revealed the mechanism of formation of delafossite CuFeO 2 from stable CuO and spinel phases at 1022 ± 2°C and its incongruent decomposition into liquid and spinel phases at 1070 ± 2°C. For the first time, a cationic off-stoichiometry of cuprous ferrite CuFe 1−y O 2−δ was unambiguous, as evidenced by two independent sets of experiments: (1) Electron probe microanalysis evidenced homogeneous micronic CuFe 1−y O 2−δ areas with a maximum y value of 0.12 [i.e., Fe/(Cu + Fe) = 0.47] on Cu/Fe gradient generated by diffusion from a perfect spark plasma sintering pristine interface. Micro-Raman provided structural proof of the existence of the delafossite structure in these areas. (2) Standard Cu additions from the stoichiometric compound CuFeO 2 coupled with high-temperature X-ray diffraction corroborated the possibility of obtaining a pure Cu-excess delafossite phase with y = 0.12. No evidence of an Fe-rich delafossite was found, and complementary analysis under a neutral atmosphere shows narrow lattice parameter variation with an increase of Cu in the delafossite structure. The consistent new data set is summarized in an updated experimental Cu−Fe−O phase diagram. These results provide an improved understanding of the stability region and possible nonstoichiometry value of the CuFe 1−y O 2−δ delafossite in the Cu−Fe−O phase diagram, enabling its optimization for specific applications.