Phase equilibria in the SrO─CuO─Cu system were determined at 1173 K from the results of X‐ray diffraction measurements using specimens annealed under low oxygen partial pressure (po2= 0.21–10−8atm). Electromotive force (EMF) measurements using ZrO2solid electrolyte cells were carried out in the ternary phase equilibria. Gibbs free energies for five reactions in the cell electrodes were summarized by equations with linear temperature dependence. The standard free energies of formation for Sr2CuO3, SrCuO2, Sr14Cu24O41, and SrCu2O2were derived. The stability conditions of all oxides are displayed in thep‐T‐xdiagram using