The phase stability, thermal expansion, electrical conductivity, and oxygen permeation of perovskite-type oxides Ba0.5Sr0.5(Co0.8Fe0.2)1 − xNbxO3 − δ (x = 0 − 0.2) have been investigated. Room-temperature X-ray diffraction of as-prepared powders indicates that in the investigated compositional range solid solutions are formed. Long-term annealing experiments both in flowing air and nitrogen, at 750 °C, demonstrate that the phase instability observed in parent Ba0.5Sr0.5Co0.8Fe0.2O3 − δ (BSCF) is suppressed already at the minimum substitution of 5 mol% of niobium for (Co, Fe). Both electrical conductivity and thermal expansion are found to decrease with increasing niobium concentration, which behaviors can be explained by defect chemical considerations, taking into account charge compensation mechanisms by doping BSCF with Nb5+ donor cations. The oxygen permeation flux of 10 mol% Nb-substituted BSCF, in the range 800–900 °C, is reduced by 10% relative to that found for parent BSCF. Switching from helium to a CO2-containing purge gas results in a severe reduction or cessation of the oxygen flux. Options are discussed to avoid undesired formation of surface carbonates.