Degradation of microstructure and phase composition of perovskite-structured Ba0.5Sr5.5Co0.8Fe0.2O3-delta membrane was investigated after exposure to CO2-containing atmospheres in the temperature range of 800-900 degrees C. A carbonate surface layer was formed on the treated membrane. Adjacent to the carbonate layer is an intermediate decomposed zone, consisting of mainly CoO and a Ba-Sr-Co-Fe oxide compound as well as (Ba. Sr)CO3 carbonate. Three different carbonate phases were observed, depending on the annealing conditions. The growth of both the carbonate top layer and the decomposed zone obeys a parabolic rate law, indicating a diffusion-controlled growth mechanism. The growth kinetics and mechanism are insusceptible to the change of partial pressure of oxygen and carbon dioxide under the given conditions. Deviation from the nominal composition was observed in this decomposed zone, which becomes more pronounced closer to the carbonate top layer. The carbonate layer was found to grow outward, which indicates outward diffusion of barium and strontium cations under their respective chemical potential gradient. Full substitution of Co by Fe in this material leads to reduced degradation rate in CO2, which is consistent with the lowering of the chemical potential of the A-site cations. The adverse effect of CO2 was found to be more pronounced at the grain boundaries of the membrane. (C) 2011 Published by Elsevier