The object of the study is the complex oxides formed in the ½ Dy2O3– BaO – ½ Fe2O3 system and the samples obtained based on these phases. Synthesis of complex oxides and X-ray analysis of all synthesized samples has been performed. The changes of oxygen content in the solid solutions were measured by thermogravimetric analysis and iodometric titration as a function of temperature. Thermal expansion, total electrical conductivity, and magnetic properties were studied for the cobalt substituted complex oxide. A subsolidus isothermal isobaric phase diagram (1100 °C, in air) for the ½ Dy2O3 – BaO – ½ Fe2O3 system has been constructed.