The electrochemical performance and stability of (La,Sr)MnO3-Y2O3-ZrO2 (LSM-YSZ) composite oxygen electrodes is studied in detail under solid oxide electrolysis cells (SOECs) operation conditions. The introduction of YSZ electrolyte phase to form an LSM-YSZ composite oxygen electrode substantially enhances the electrocatalytic activity for oxygen oxidation reaction. However, the composite electrode degrades significantly under SOEC mode tested at 500 mA cm(-2) and 800 degrees C. The electrode degradation is characterized by deteriorated surface diffusion and oxygen ion exchange and migration processes. The degradation in electrode performance and stability is most likely associated with the breakup of LSM grains and formation of LSM nanoparticles at the electrode/electrolyte interface, and the formation of nano-patterns on YSZ electrolyte surface under the electrolysis polarization conditions. The results indicate that it is important to minimize the direct contact of LSM particles and YSZ electrolyte at the interface in order to prevent the detrimental effect of the LSM nanoparticle formation on the performance and stability of LSM-based composite oxygen electrodes. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.