Oxygen ionization and diffusion were visualized at cathode/interlayer/electrolyte interfaces by isotopic oxygen labeling (O-16/O-18 exchange) in real flatten tube solid oxide fuel cells. Three-flatten tube cell-stacks were exposed to O-18(2) under fuel cell operation conditions at different current densities (J = 0, 0.10, and 0.25 A/cm(2) at a temperature of 650 degrees C), and the distribution of O-18 was observed at the quenched samples by secondary ion mass spectrometry (SIMS). An increase of O-18 concentration at LaFeO3 based cathode and a peak O-18 concentration formation around the CeO2-based interlayer were observed. The increase of O-18 concentration at the LaFeO3 based cathode is attributed to an increase of oxygen surface exchange rates under polarization. The peak O-18 concentration increased with current densities through the interlayer/electrolyte interfaces. The formation of O-18 peak can be related to the formation of a slow O-18-diffusion layer at the interlayer/electrolyte interfaces. Possible mechanism for formation of the O-18 peak around the interlayer/electrolyte interfaces was considered. (c) 2012 Elsevier B.V. All rights reserved.