The effects of long-term operation on the oxygen reduction reaction were visualized at the cathode/interlayer/electrolyte interface in Solid Oxide Fuel Cells (SOFCs) by using a stable isotope oxygen (O-18) labeling technique with Secondary Ion Mass Spectrometry (SIMS). Real flat-tube stacks were exposed to O-18(2) before and after long-term operation time (800 hours) for "direct observation" of oxygen/oxide ion motions. The trace of oxygen motions were labeled under fuel cell operation conditions (current density of 0.25 A/cm(2) at 650 degrees C). The SIMS images were taken during electronic shower by E-gun to avoid the charge-up during the measurements. The O-18 incorporation and O-18(2)-diffusion were visualized in a "frozen state" by this technique. High signal counts of O-18-were obtained at the CeO2-interlayer/electrolyte interface, whereas higher O-18-signal counts were observed inside the CeO2-interlayer at the initial condition. A slow diffusion layer of O-18(2)-can be formed after long-term operation and it affects the distribution of high O-18-signal areas at the interfaces. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.069208jes]