Secondary phase formation poses a major obstacle in the application of the oxygen-conducting perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) in the intermediate temperature range (700 – 800 °C). Doping with suitable transition metals like Sc, Zr, and Y was investigated to stabilize the cubic BSCF phase. The material was studied by scanning and transmission electron microscopy. This includes the determination of the chemical composition (EDXS), dopant lattice sites (ALCHEMI) and changes of the Co-valence state (EELS). Y-doping was found to strongly reduce secondary phase formation in BSCF.