The stability of the solid oxide fuel cell (SOFC) cathode material Ba0.5Sr0.5Co0.8Fe0.2O3-delta in CO2-containing atmospheres (4 x 10(-4) <= pCO2/bar <= 5 x 10(-2)) is investigated by precision thermogravimetry (TG) and mass spectrometry (MS) as a function of temperature (20 <= T/degrees C <= 950). The desorption of O-2 and CO2 from samples with different pretreatments is compared. Oxygen exchange at 300 < T/degrees C < 700 is significantly impaired by CO2-containing atmospheres. At 600 degrees C the kinetics of carbonate formation in CO2-rich atmosphere is described by a linear-parabolic rate law. A pronounced dependence of the rate constant on the CO2 content is suggested. Temperature cycles at 0 <= pCO(2)/bar <= 5 x 10(-2) and pO(2) = 0.2 bar, which show effects due to oxygen exchange of the perovskite and due to CO2, are analyzed based on reference experiments in a CO2-free atmosphere, and under consideration of the TG-MS results. The decomposition temperature of the carbonate in contact with an atmosphere of pCO(2) = 5 x 10(-2) bar amounts to 807 degrees C. Complementary, the impact of carbonate formation on the oxygen exchange kinetics is investigated by conductivity relaxation measurements. A severe degradation of the surface oxygen exchange coefficient is observed after 3-10 days of exposure to ambient air. Regeneration and activation of the oxygen exchange kinetics is demonstrated after treatment in a CO2-free atmosphere at 825 degrees C. (C) 2008 The Electrochemical Society.