The phase diagram of the system Sr–Cu–O has been determined at 1173 K. Several compositions in the ternary system were quenched after equilibration, and the phases present were identified by optical microscopy, X‐ray diffraction, and electron proble microanalysis. Four ternary compounds, SrCu2O2, Sr2CuO3, SrCuO2+δ, and Sr14Cu24O41, have been identified. Values for the nonstoichiometric parameter δ in SrCuO2+δcorresponding to the different three‐phase fields have been determined using the hydrogen reduction technique. The thermodynamic properties of all the ternary oxides were determined using solidstate cells incorporating either oxide or fluoride electrolyte. The results from both types of cells were internally consistent. A discrepancy in the recent literature regarding thermodynamic properties of Sr2CuO3has been resolved. The compound Sr14Cu24O41has been identified in some of the earlier investigations as Sr3Cu5O8+z˙Using a novel variation of the galvanic cell technique, in which the emf of a cell incorporating a fluoride electrolyte is measured as a function of oxygen potential of the gas phase in equilibrium with the condensed phase electrode, the formula Sr14Cu24O41has been confirmed. The compound does not have significant nonstoichiometry. The phase relations in the system Sr–Cu–O have been evaluated as a function of temperature and oxygen partial pressure from the the