We have examined the long-term durability of a La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)–samaria-doped ceria (SDC) composite oxygen electrode with SDC interlayer for reversible solid oxide cells (R-SOCs). A symmetrical cell with the configuration: LSCF–SDC|SDC interlayer|yttria-stabilized zirconia (YSZ) electrolyte|SDC interlayer|LSCF–SDC, was operated at 900°C and a constant current density of 0.5 A cm⁻² with the top electrode as the anode (O₂ evolution). The IR-free overpotentials at both the anode and cathode were virtually constant during 5500 h of operation. The value of ohmic resistance at the anode side (R_A) increased slightly, whereas that at the cathode side (R_C) increased markedly. The I–E performance of the bottom electrode (operated as the cathode), that was measured from −1.0 to 1.0 A cm⁻² every 1000 h, degraded specifically at high current densities. It was found that the thickness, pore size, and porosity in both electrodes were unchanged, but the distribution of the Sr component changed markedly at both the LSCF–SDC/SDC interlayer and SDC interlayer/YSZ interfaces. While the diffusion of the Sr component from the anode was limited within the SDC interlayer, the Sr component from the cathode reached the SDC interlayer/YSZ interface, which could increase the R_C, likely due to the formation of SrZrO₃. However, the diffusion rates of Sr were found to be noticeably slowed down at dense portions of the SDC interlayer. Hence, it is essential to prepare a dense, uniform SDC interlayer to improve both the durability and performance of R-SOCs.