One of the most promising mixed ionic electronic conducting oxygen transport ceramic materials is the perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF 5582). BSCF membrane tubes are characterized after operation in a demonstrator unit with respect to microstructure and failure origin. A comparison of the microstructures of the operated tubes with reference material after specified thermal/environmental treatment permitted a reconstruction of the axial temperature profile. On the contrary to the annealing studies, that revealed the formation of hexagonal phase only in air, the studies of operated tubes confirmed the existence of hexagonal phase also on the vacuum side, which appears to be a result of oxygen transport through the membrane. Fractographic investigations suggest that the thermal exposure of BSCF under application relevant conditions can lead to crack initiation and eventually failure, which could be avoided by a modification of the demonstrator operation procedure. (C) 2014 Elsevier B.V. All rights reserved