Mixed ionic-electronic conductors and, among those, in particular Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF5582) with high oxygen ion conductivity are of great interest for oxygen separation membranes. To study the decomposition of BSCF5582 under application-relevant conditions, BSCF5582 bulk samples were annealed at temperatures between 700 degrees C and 1000 degrees C and investigated by X-ray diffractometry and various electron microscopic techniques. Beside the verification of the formation of a hexagonal phase, an additional phase with plate-like morphology and trigonal crystal structure is observed. While the crystallization of the hexagonal phase is limited to temperatures below 830 degrees C, the plate-like phase is detected up to 900 degrees C annealing. The chemical composition of the hexagonal and plate-like phases was analyzed by energy dispersive X-ray spectroscopy. The plate-like phase is characterized by a cobalt enrichment and strontium depletion whereas the barium concentration remains unchanged with respect to the cubic BSCF matrix. The kinetics of the decomposition at 800 degrees C of the cubic phase was studied by annealing treatments up to 1350 h. The plate-like phase shows fast precipitation whereas the formation of the hexagonal phase was found to be significantly slower. (C) 2011 Elsevier B.V. All rights reserved.