We report on the experimental determination of the phase diagram of the quasi-binary subsystem CuO-BaCuO2 with the aid of differential scanning calorimetry (DSC). Al2O3 crucible material shows severe corrosion and influence on the phase relationships. Crucibles made from zirconium oxide stabilized with yttrium oxide exhibit less chemical interaction. Due to the sluggish reactions within the system and the unavoidable corrosion, the parameters of the DSC runs must be carefully optimized. The oxygen content in the atmosphere changes the temperature of the liquids line and the eutectic solidification. Moreover, it influences the formation of monovalent copper oxide. BaCuO2 is shown to melt congruently. Reasons for differences between the given phase diagram and those reported in the literature are discussed.