The oxygen reduction active sites were visualized around the O 2/SOFC cathode/electrolyte triple phase boundaries (TPB) by the 16O/18O exchange techniques and secondary ion mass spectrometry (SIMS) analysis. The higher 18O concentration is observed on the cathode top surfaces (La0.9Sr0.1MnO 3-mesh, Au-mesh, and Ag-porous), which suggested the promotion of oxygen adsorption and oxygen surface exchange at the cathode. The oxygen diffusion through the bulk of cathode occmTed at the La0.9Sr 0.1MnO3-mesh and the Ag-porous cathodes, not at the Au-mesh cathode. On the YSZ surfaces after removing the cathode, the active sites for oxygen incorporation were analyzed by SIMS. The active sites for oxygen incorporation were at the La0.9Sr0.1MnO 3/YSZ interface as well as the TPB. On the other hand, the active sites for oxygen incorporation are limited to the TPB in the case of the Au-mesh removed YSZ surface. From the SIMS analysis, the expansion of the active sites for oxygen incorporation is less than a few μm from the TPB lines.