Performance and durability of Ni-YSZ supported solid oxide electrolysis cells (SOECs) for co-electrolysis of H2O and CO2 at high current density was investigated. The cells consist of a Ni-YSZ support, a Ni-YSZ electrode, a YSZ electrolyte, and an LSM-YSZ electrode. The durability was examined at 800 degrees C and electrolysis current densities of -1 or -1.5 A/cm(2) with 60% reactant (H2O + CO2) utilization. The cell voltage degradation showed a strong dependence on the electrolysis current density. Electrochemical characterization of the cells showed that the degradation was mainly related to the LSM-YSZ electrode when operated at -1 A/cm(2), whereas at increased current density (-1.5 A/cm(2)), both the Ni-YSZ and the LSM-YSZ electrode degraded. The initial degradation (0 - 200 h) is mostly due to increasing of polarization resistance, while the long term degradation is mainly caused by increasing of serial resistance. (C) 2013 The Electrochemical Society. All rights reserved.