Commercialized Ni-YSZ/YSZ/LSM-YSZ solid oxide cells with the dimension of 10 cm x 10 cm are investigated in the H2O-CO2 co-electrolysis mode at 800 degrees C. The aim is to understand the occurrence of the individual CO2 electrolysis during polarization. The electrochemical impedance spectroscopy is combined with the online gas chromatography in order to reveal the possible processes. Several inlet gas flow scenarios are selected and tested at various current density. As a theoretical evidence, the electrolysis voltage comparison of H2O/H-2 and CO2/CO is presented as well. The results suggest the co-electrolysis process starts with the individual H2O electrolysis which has a relatively lower reaction barrier. The higher-reaction-barrier CO2 electrolysis is activated and concurrent with the H2O electrolysis if more electric energy is applied and that's when the real sense of H2O-CO2 co-electrolysis occurs. (C) 2019 Elsevier Ltd. All rights reserved.