Phase stability and chemical reactivity of (La0.6Sr0.4)(0.99)CoO3-delta (LSC64) was tested in oxidative (pO(2) = 0.21 atm) and slightly reducing conditions (pO(2) similar to 10(-5) atm), as well as in carbon dioxide (pO(2) similar to 10(-4) atm) to evaluate the material performance for oxygen separation technologies. Thin film LSC64 oxygen separation membranes (20-30 mu m) were manufactured and electrochemical performance was evaluated at a range of temperatures with either nitrogen or CO2 purged on the permeate side of the membrane. Material stability was also investigated by high temperature X-ray diffraction, TGA and conductivity measurements in air, N-2 and CO2. Under mild reduction LSC64 partly decomposes to a K2NiF4-type phase (i.e. (La,Sr)(2)CoO4), and Co-oxide, and under high pCO(2) forms SrCO3. The latter is found to impair membrane performance. Electrical properties and oxygen permeation (jO(2)) in thin membranes depend on the thermal and chemical history of the samples. A flux of 4-6 Nml min(-1) cm(-2) in the temperature range of 800-900 degrees C was demonstrated for optimized membranes and conditions. (C) 2012 Elsevier B.V. All rights reserved.