The mixed ionic-electronic conductor La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) is a candidate for use as the membrane material in the pressure-driven air separator. However the long-term stability of these materials is a major concern for the commercial realization of these devices. In an attempt to assess the stability of the material under realistic operating conditions for such a system, samples of LSCF were exposed to atmospheres of H2O/CO2/O-2 at 750 degrees C. Degradation of the material was studied using Raman spectroscopy and X-ray diffraction. Oxygen isotope exchange combined with secondary ion mass spectrometry was used to observe the effect of the presence of CO2 and H2O on the oxygen diffusivity and surface exchange. It was observed that oxygen surface exchange was actually enhanced in the presence of CO2 and H2O during the duration of the study, although the oxygen diffusivity remained the same. The presence of volatile chromia species was found to significantly increase the rate of degradation of the ceramic.