Mixed ionic-electronic conducting (MIEC) ceramics as oxygen transport membranes (OTMs) can provide high oxygen permeation rates at comparably low energy demands. For this purpose, Ba . Sr . Co . Fe .2O d (BSCF) shows the best performance under ideal operating conditions. Thermal and chemical stability investigations, electrical behavior s(T,pO2,t), and oxygen exchange parameter extraction by means of electrical conductivity relaxation resulted in a far better understanding of the BSCF system.< ...