In this study, single cells employing a La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) cathode were operated with a supply of humidified oxygen to the cathode at 1000 degrees C for 100 h to investigate the influence of water vapor on the performance and microstructure of LSCF cathode. When a gaseous mixture of 20% H2O-80% O-2 was supplied to the cathode, the performance of LSCF cathode continuously lowered during a discharge at 300 mA cm(-2) for 100 h. Then, the microstructures of surface and cross-section of LSCF cathode were observed by scanning electron microscopy. The surface morphology was drastically changed by the discharge operation. A SrO layer was formed at the outermost surface of cathode, indicating that the strontium segregation was accelerated by water vapor. In response to this phenomenon, the formation amount of cobalt- and/or iron-based oxides enlarged inside the electrode. These microstructural and phase changes would be responsible for the performance deterioration of LSCF cathode. (C) The Author(s) 2019. Published by ECS.