It is shown that an excess of Dy in A-site (in amount of 0.2 in formula units) allows synthesizing a single-phase perovskite-related oxide with the 123-type structure at 1100 °C in air. Cobalt ions completely fill the octahedrally coordinated positions in the structure of 123-type Dy1.2Ba1.8Fe1.8Co1.2O8-δ, while iron together with excess Co is in the pyramidal oxygen coordination. The oxygen content at room temperature of 7.86 decreases with temperature. A slight increase in the slope of thermal expansion observed at T >200 °C. The electrical conductivity, carried out mainly by holes, increases non-linearly with temperature, reaching a maximum value 14.8 S ⋅ cm−1 at 900 °C. Magnetic measurements allow assuming that Fe3+ are in low spin state, all Co2+ ions are in high spin, and Co3+ can be split into several states depending on coordination. © 2023 Wiley-VCH GmbH.