In the first part of this study, the oxygen deficiency, δ, in Ba0.5Sr0.5Co0.8Fe0.2O3 - δ (BSCF) was measured by means of thermogravimetry as a function of oxygen partial pressure, p(O2), in the range of 1.1×10−6 < p(O2)/% < 41.67 at elevated temperatures ranging 873 ≤ T/K ≤ 1073. It was shown that δ becomes more pronounced with increasing T and with decreasing p(O2). The isotherms δ vs. p(O2) were determined. The second part of this study relates to the reaction of CO2 with Ba0.5Sr0.5Co0.8Fe0.8O3-δ perovskite oxide in the absence and presence of O2 at temperatures ranging from 673 to 973 K also by thermogravimetry. The reactivity of CO2 with BSCF increased with increasing temperature and increasing exposure to CO2. Reaction of CO2 with BSCF was described by a equilibrium reaction isotherms. The results of X-ray diffractometry evidenced that the exposure to CO2 leads to the formation of carbonates.