A vacuum-annealed La0.6Ca0.4CoO3-x, was consecutively oxygenated in air at temperatures decreasing from 800 to 100 degreesC, and its electrocatalytic activities for oxygen reduction and evolution were then measured as a function of the oxygenation temperature. The valence of Co cation, changing between +2 and +3, was found susceptible to annealing either in vacuum or air. The catalytic activities initially decrease monotonically as the oxygenation temperature was decreased from 800 to 300 degreesC, as a result of increasing oxygen content, and then rise abruptly with the oxygen reduction activity reaching a maximum at 200 degreesC and the oxidation activity at 150 degreesC. X-ray photoelectron spectroscopy analysis indicated that the enhancements by the low-temperature oxygenation involved increased OH coverage and less charged cations at surface. The results clearly reveal the importance of the post-calcination annealing process for optimizing the performance of La0.6Ca0.4CoO3-x in air electrode applications. (C) 2003 Elsevier Science Ltd. All rights reserved.