The application of mixed ionic-electronic conducting (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-delta (BSCF) as gas separation membrane is up to now hampered by secondary phase formation which impairs the excellent oxygen permeation properties of this material. In this work, we have studied the impact of grain size and A/B-cation ratio on secondary phase formation in BSCF and Y-doped (Ba0.5Sr0.5)(Co0.8Fe0.2)(0.9)Y0.1O3-delta (BSCF10Y) by electron microscopic techniques before and after long-term thermal exposure at an application-relevant temperature (similar to 760 A degrees C). A large content of secondary phases is found in samples with small grain sizes because grain boundaries provide nucleation sites for secondary phases. Higher sintering temperatures increase the grain sizes and substantially reduce the content of secondary phases. Variations of the A/B-cation ratio between (Ba0.5Sr0.5)(0.95)(Co0.8Fe0.2)O3-delta and (Ba0.5Sr0.5)(1.05)(Co0.8Fe0.2)O3-delta do not lead to a change of the composition of the cubic BSCF phase but changes the volume fraction of Co3O4 precipitates which are already formed during sintering. BSCF with an excess of A-site cations contains the smallest overall amount of secondary phases in undoped BSCF due to the minimization of Co3O4 precipitation during sintering and the reduction of nucleation sites for other secondary phases at application-relevant temperatures. Secondary phase formation in BSCF10Y can be almost completely suppressed due to the stabilization of the cubic BSCF phase by Y-doping and large grain sizes after high-temperature sintering.