The influence of operating temperature and fuel/air utilization on long-term chemical stability and cell performance degradation is comprehensively investigated and reported for a segmented-in-series tubular solid oxide fuel cell (SOFC), with a (La0.5Sr0.25Ca0.25)MnO3 (LSCM) cathode and a (Ce0.8Sm0.2)O-2 (SDC) cathode interlayer, under development for large-scale power plants combined with SOFCs. During three kinds of durability tests for 5000 hours, the average degradation rates of the cell-stacks were around zero, well meeting a tolerant cell voltage degradation rate target of 0.25% per 1000 hours, corresponding to the 10% cell voltage degradation for 40,000 hours. The electrochemical performance was stable at high operating temperature, although the porosity of the SDC cathode-interlayer decreased due to Mn and Ca diffusion from the LSCM cathode. On the other hand, the cell voltage noticeably decreased at lower operating temperature, below 800 degrees C. Detailed observation of the microstructure and elemental distribution after durability testing revealed that a dense layer formed between the LSCM cathode and the SDC interlayer at low temperature, consisting of La, Ca, and Mn. The degradation in cell performance is attributed to the formation of this dense layer, preventing oxygen supply to the electrode reaction sites. (C) 2013 The Electrochemical Society. All rights reserved.