The phase relationships of the BaO–CaO–CuO ternary system at 850 °C in air was investigated by X-ray powder diffraction, scanning electron microscopy and electron probe microanalysis techniques. The samples were prepared by solid state reaction method. The existence of three binary oxide compounds, Ba 2 CuO 3 , BaCuO 2 and Ca 2 CuO 3 was confirmed and no other intermediate phases were found in this system at 850 °C. The isothermal section consists of six single-phase regions, nine two-phase regions and four three-phase regions. No carbonate was detected. However, barium dioxide, BaO 2 , was found in BaO-rich corner of this system. At 850 °C, Ba 2 CuO 3 keeps its high temperature structure, being tetragonal with a = 1.2978 nm and c = 0.3992 nm. BaCuO 2 has a cubic crystal structure with a = 1.82855 nm, and Ca 2 CuO 3 is orthorhombic with a = 0.32573 nm, b = 0.37769 nm and c = 1.2234 nm.