In this study, a ten-layer SOC-stack, consisting of large planar electrolyte supported cells (ESC) and CFY interconnects, was operated in both fuel cell and electrolysis modes considering steam-, co-, and CO2-electrolysis. Different gas mixtures containing H-2, H2O, CO, CO2 , and N-2 were applied for this purpose, and an extensive analyses were performed at two temperatures, 800 degrees C and 850 degrees C. Under these operating conditions, independent Electrochemical Impedance Spectroscopy (EIS) measurements were performed on the stack as a whole, and several cells within the stack concurrently. In order to analyze the losses that occurred during operation, measured impedance values were analyzed, and an electrical equivalent circuit (EEC) for ESCs was applied to the measured impedance spectra. Finally, the electrochemical impedance measurements were performed at both OCV and elevated current densities. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.