Recently, segment-in-series tubular solid oxide fuel cells (SOFCs) fabricated by Mitsubishi Hitachi Power System (MHPS) exhibited excellent performance for more than 30 000 h with a degradation rate of −0.10% kh⁻¹; cells consisted of Ni–yttria-stabilized zirconia (YSZ) cermet anode, YSZ electrolyte, Sm-doped ceria (SDC) cathode interlayer, and (La,Sr,Ca)MnO₃ (LSCM) cathode. This degradation rate was smaller than those for conventional cells with a LSCF cathode/Gd-doped ceria interlayer/YSZ electrolyte configuration. The reason for such high durability has not been elucidated sufficiently, though the microstructure at the cathode side changed upon discharge operation as in the case of conventional cells. In this study, then, the constituent materials as well as the cell configuration, which were presumed in the vicinity of the cathode/interlayer interface after prolonged operation of real cell stacks, were modeled. Concretely, the composites of (Sm,Ca)MnO₃–MnO_x were prepared as model cathodes and their electrocatalytic activity for the oxygen reduction reaction was studied with varying a manganese content. Furthermore, the electrochemical activity, the phase stability, and the microstructural stability of (Sm,Ca)MnO₃–MnO_x were analyzed in detail under the coexistence of SDC ((Sm,Ca)MnO₃–MnO_x–SDC composite system).