Surface activation and bulk transport of oxygen in dense samples of La0.2Sr0.8BCr0.2Fe0.8O3-d were studied with Ion Exchange/Depth Profiling (IEDP) techniques. Both high oxygen activity (20% O-18(2)) and low oxygen activity (CO/(CO2)-O-18) conditions were studied at 1023 and 1123 K. Mass spectrometry of the gas phase during the isotope infusion allows the resolution of two pathways for isotope exchange with the solid. Redox exchange, which is significant for the performance of oxygen transport membranes is relatively slow compared with rapid direct isotope exchange between the solid and CO2. The rates of this redox process are somewhat lower than molecular oxygen activation in 20% O-2 at the same temperature and thus could be a kinetically significant factor in steady state operation of oxygen transport membranes. The oxygen bulk mobilities were similar in the two conditions. (C) 2004 Elsevier B.V. All rights reserved.