The preferential reaction path for oxygen isotope exchange was investigated on a large-grained polycrystalline (La0.6Sr0.4)CoO3 With (LaSr)(2)CoO4 grains dispersed as the secondary phase. The isotope exchange experiment was performed at 773 K in O-18 enriched oxygen of 0.2 x 10(5) Pa. Three-dimensional imaging of Secondary Ion Mass Spectrometry (SIMS) was carried out to visualize the isotope distribution. The SIMS image suggested that there are fast oxygen-incorporation paths along the heterophase boundary of (La0.6Sr0.4)CoO3 / (La,Sr)(2)CoO4. The surface exchange coefficient, k for the heterophase boundary was roughly estimated by analyzing the depth profile of O-18. The calculated k was 8 X 10(-6) cm s(-U1), which is larger than the value of k for the surface of single phase La0.6Sr0.4CoO3 by three orders of magnitude. (C) 2007 Elsevier B.V All rights reserved.