The decomposition mechanism of manganese-containing 10 mol% Sc2O3 and 1 mol% CeO2 co-doped ZrO2 (10Sc1CeSZ) electrolyte during long-term operation of a cathode-supported tubular-type solid oxide fuel cell (SOFC) was investigated. With Fe2O3, TiO2, Na2CO3. and MgO applied respectively on an electrolyte surface, cell operation tests were conducted at 1000 C for 270 h. Additionally, an annealing test using manganese-containing 10SclCeSZ pellets was conducted under a fuel atmosphere to clarify the influence of P(O-2) distribution in the electrolyte. After operation or annealing, samples were analyzed using SEM/EDX and Raman spectroscopy. Decomposition occurred on the electrolyte surface where Fe2O3 or TiO2 had been applied, but it did not occur on annealed pellet surfaces. Because reactive products containing manganese oxides were detected on surfaces to which Fe2O3 or TiO2 had been applied, results revealed that decomposition of this type requires a nucleation site of Mn3O4 precipitation and a high-p(O-2) region near the electrolyte surface. On the surface with Na2CO3 or MgO, the grain boundaries were particularly damaged. Pellet surfaces were also damaged. Results suggest that the electrolyte surface decomposition caused by manganese oxide. precipitation is a new phenomenon that can be regarded as different from decomposition caused by an alkali metal such as Na2CO3. (C) 2012 Elsevier B.V. All rights reserved.