The preferential site for oxygen isotope exchange was investigated on layered films of La2-xSrxCoO4 (x=0.5,1.0)/La0.6Sr0.4CoO3 prepared by pulsed laser deposition (PLD) on a polycrystalline Ce0.9Gd0.1O1.95. An isotope exchange experiment was performed at 773 K in O-18-enriched oxygen of 0.2x10(5) Pa. Three-dimensional imaging of secondary ion mass spectrometry (SIMS) was carried out to visualize the isotope distribution. The SIMS images suggested that there are fast oxygen-incorporation paths along the hetero-phase interface of La2-xSrxCoO4/La0.6Sr0.4CoO3. The results have reproduced our former findings on the fast oxygen surface exchange with La2-xSrxCoO4/La0.6Sr0.4CoO3 composite polycrystals. (C) 2008 The Electrochemical Society.