The decomposition mechanism of manganese-containing 10mol% Sc 2 O 3 and 1mol% CeO 2 co-doped ZrO 2 (10Sc1CeSZ) electrolyte during long-term operation of a cathode-supported tubular-type solid oxide fuel cell (SOFC) was investigated. With Fe 2 O 3 , TiO 2 , Na 2 CO 3 , and MgO applied respectively on an electrolyte surface, cell operation tests were conducted at 1000°C for 270h. Additionally, an annealing test using manganese-containing 10Sc1CeSZ 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 Fe 2 O 3 or TiO 2 had been applied, but it did not occur on annealed pellet surfaces. Because reactive products containing manganese oxides were detected on surfaces to which Fe 2 O 3 or TiO 2 had been applied, results revealed that decomposition of this type requires a nucleation site of Mn 3 O 4 precipitation and a high-p(O 2 ) region near the electrolyte surface. On the surface with Na 2 CO 3 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 Na 2 CO 3 .