Dense mixed-conducting composite materials of BC (La(0.9)Sr(0.1)CoO(3-delta) or La(0.6)Sr(0.4)CoO(3-delta)) and CGO (Ce(0.9)Gd(0.1)O(2-delta)) prepared by conventional powder mixing and sintering have been studied for possible application as oxygen separation membranes. The composites showed high relative density (above 95%), low background gas leakage and also good electrical conduction. The percolation threshold of the electronic-conducting component was determined to be approximately 20 vol.% for both LSC compositions. Isotopic exchange and depth profiling by secondary ion mass spectrometry was used to investigated the oxygen tracer diffusion (D*) and surface exchange coefficient (k*) of the composites. Composites exceeding the electronic percolation threshold, showed high oxygen diffusivities and surface exchange properties and intermediate thermal expansion thus combining the benefits of their constituent materials. (C) 2010 Elsevier B.V. All rights reserved.