Solid oxide fuel cells with a configuration of (La,Sr)(Co,Fe)O3-delta cathode/doped ceria interlayer/zirconia-based electrolyte have been extensively studied to elucidate the degradation mechanism. Various degradation factors were suggested, such as the formation of highly-resistive SrZrO3 phase, and the reduction in active reaction sites because of the agglomeration of constituent materials. Among them, however, the influence of the ceria-zirconia solid solution formation at the doped ceria interlayer/zirconia-based electrolyte on the cell performance has not been elucidated sufficiently. In this study to achieve a comprehensive understanding of degradation phenomena at the cathode side, the chemical information of constituent materials, as well as the microstructural parameters, were analyzed for single cells before and after discharge operation. Especially, the ionic conductivity of solid solutions formed in the (La,Sr)(Co,Fe)O3-delta/Gd2O3-CeO2/Y2O3-ZrO2 system was investigated in detail to clarify the ionic conductivity profile at the Gd2O3-CeO2/Y2O3-ZrO2 interface. (C) The Author(s) 2019. Published by ECS.