Active sites for oxygen reduction were investigated at the interface of O-2/La0.9Sr0.1MnO3-x (LSM)/yttria-stabilized zirconia (YSZ). Isotopic oxygen (O-16/O-18) exchange under cathodic polarization and secondary-ion mass spectrometry (SIMS) analysis were examined to visualize the oxygen reduction active sites. The LSM mesh pattern electrode was prepared to define the contact area of LSM/YSZ. Under cathodic polarization, oxygen can diffuse through the dense LSM via oxygen vacancy, which promotes the electrode reaction. By SIMS imaging technique, the active sites for oxygen reduction were clearly determined as spots around the O-2/LSM/YSZ three-phase boundary. The Line analysis of the SIMS image enabled us to draw a contour map of the O-18 concentration in the cross section of YSZ. The diffusion paths were clearly visualized in the contour map. The width of the active sites for oxygen reduction is estimated to be less than 1 mu m under the examined condition.