YSZ-electrolyte supported solid oxide electrolyzer cells (SOECs) using LSM-YSZ oxygen electrode but with three types of hydrogen electrode, Ni-SDC, Ni-YSZ and LSCM-YSZ have been fabricated and characterized under different steam contents in the feeding gas at 850 degrees C. Electrochemical impedance spectra results show that cell resistances increase with the increase in steam concentrations under both open circuit voltage and electrolysis conditions, suggesting that electrolysis reaction becomes more difficult in high steam content. Pt reference electrode was applied to evaluate the contributions of the hydrogen electrode and oxygen electrode in the electrolysis process. Electrochemical impedance spectra and over potential of both electrodes were measured under the same testing conditions. Experimental results show that steam contents mainly affect the behavior of the hydrogen electrode but have little influence on the oxygen electrode. Further, contribution from the hydrogen electrode is dominant in the electrolysis process for Ni-based SOECs, but this contribution decreases for LSCM-based SOECs. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3615992] All rights reserved.