Compositions of La1‐xBaxCuO3, where x ranges from 0.0 to 0.5, were fired in air, oxygen‐enriched air, and oxygen. Studies show that BaO cannot make a solid solution with LaCuO3without changing the basic structure. The resulting phases, in all attempts, were the binary compounds La2CuO4, La1‐xBaxCuO3‐δ(x= 0.2 to 0.5), or their mixtures. All samples showed metallic conductivity. Extra oxygen in the reaction atmosphere appeared to encourage the formation of the LaCuO3‐based phases of La1‐xBaxCuO3‐δ(x= 0.2 or 0.5). We provide a defect‐chemical and thermodynamical explanation for