Oxygen reduction active sites were investigated at the interface of O-2/La-0.9,Sr-0.1,MnO3-x (LSM)/yttria-stabilized zirconia (YSZ) for three different overvoltages of cathodic polarization (n = - 0.336 V, n = - 0.185 V, and n = - 0.090 V versus reference electrode). Isotopic oxygen (O-16/O-18) exchange under cathodic polarization and secondary ion mass spectrometry (SIMS) technique were examined to visualize the oxygen incorporation/reduction active sites. The LSM mesh pattern electrode was prepared to define the area and length of contact between LSM mesh and YSZ. Under cathodic polarization of n = - 0.336 V, oxide ions can diffuse through the LSM to take part in the electrode reaction. From the SIMS images of YSZ surface, the active sites for oxygen reduction/incorporation are distributed in spots at the position where the LSM mesh was attached. Especially, O-2/LSM/YSZ three phase boundary (TPB) is the main active site for oxygen incorporation/ reduction, although some spots of high O-18 concentration were observed at the center of the LSM mesh part. Under the lower polarization (n = - 0.185 V, and n = - 0.090 V), oxide ions did not diffuse through the LSM mesh. SIMS imaging analysis of YSZ surface showed high O-18 concentration spots around the center of the LSM mesh part. The distribution of ionic currents at the surface of YSZ affects the distribution of active sites for oxygen incorporation. (C) 2000 Elsevier Science B.V. All rights reserved.