H2O-CO2 electrochemical conversion in solid oxide electrolysis cells (SOECs) is one of the efficient ways to reduce CO2 emission and to simultaneously store the renewable power. In this study, H2O-CO2 co-electrolysis performance and mechanisms in solid oxide electrolysis button cells at different operating temperature (550-750 degrees C) is tested. The results indicated that the co-electrolysis performance for Ni-YSZ/ScSZ/LSM-ScSZ electrolysis cell increases significantly with temperature. The mass transfer gradually became the rate-determining step of the whole electrodes process below 750 degrees C. By substituting half of the carrier gas to CO2 for the H2O electrolysis system, the electrochemical performance of SOEC became worse, lied between that of H2O and CO2 electrolysis, and was slightly closer to H2O electrolysis at 750 degrees C. CH4 is only detected in the gas products from the reactant composition of 28.6%H2O + 14.3%CO2 + 57.1%Ar, when the operating voltage of the electrolysis cell is higher than 2 V. The CH4 production can be significantly promoted by electricity and can be effectively suppressed by impregnating the anti-carbon deposition catalyst Ru in porous Ni-YSZ cathode. The reactions between carbon element on the electrode surface and hydrogen C(s) + 2H(2) -> CH4 is proposed as one of reaction pathways for CH4 production in H2O-CO2 high temperature co-electrolysis process. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.