Mechanisms for decomposition of 10 mol%Sc2O3 and 1 mol%CeO2 co-doped ZrO2 (10Sc1CeSZ) electrolytes of a cathode-supported tubular-type SOFC during the long term operation for >= 100 h have been investigated by analyzing SEM. Raman spectroscopy and XPS of decomposed samples and also by additional examination for the effects of impurities (MnO2, Al2O3 and Si) on the decomposition of electrolyte. The valence of manganese at the decomposed parts in the fuel side was found to become higher after the long term operation. This leads to the driving force for the decomposition of 10Sc1CeSZ into two phases by the large oxygen potential gradient developed on the surface in the fuel side. The kinetic factors for formation of new phases are considered to be as follows; the Si component in the fuel can be a trigger to form MnSiO3 which worked at the nucleation points, and the valence changes of manganese in the electrolyte proceed the decomposition cycles consisting of two phase separation in the zirconia, pulverization of manganese-rich phase and detachment of zirconia-based oxides. (C) 2011 Elsevier B.V. All rights reserved.