Dense (> 95%) interpenetrating phase composites of silver and gadolinium-doped ceria (CGO) were prepared by silver plus copper oxide (Ag-6 mol% CuO) liquid metal infiltration of porous CGO pre-forms. The composites showed high relative density (above 95%), negligible gas leakage and high electronic conductivity when the volume fraction of silver was approximately 14 vol.% or greater. Oxygen isotopic exchange and depth profiling by secondary ion mass spectrometry was used to determine the oxygen tracer diffusion (D*) and surface exchange coefficient (k*) of the composites. Composites with 14 vol.% silver exhibited similar solid state oxygen diffusivity and thermal expansion to CGO, combined with much faster surface exchange kinetics. These results indicate that such composites are promising for further development as passive oxygen separation membranes. (C) 2011 Elsevier B.V. All rights reserved.