The depth profiles of secondary ion mass spectrometry (SIMS) have been applied to analyze the distribution of 180 and metal oxide ions precisely at La0.6Sr0.4Co0.2Fe0.8O3 - delta/Gd0.1Ce0.9O2 - x (GDC)/Y0.15Zr0.85O2 - y (YSZ) interfaces. O-18 concentration peak was identified only under cathodic polarization with a constant current density of 0.071 A cm(-2) at 973 K under O-18(2) atmosphere. The O-18 peak position was corresponded to the Sr condensed zone where SrZrO3 was formed at the GDC/YSZ interfaces. The SrZrO3 formation and the O-18 concentration profile pile-up are correlated to each other. (C) 2014 Elsevier B.V. All rights reserved.