The oxygen permeability of a dual-phase membrane, Ce0.9Gd0.1O2 - delta (GDC)/La0.6Sr0.4Co0.2Fe0.8O3 - delta (LSCF) with a 50/50 vol.% composition, has been systematically investigated with varying coating materials and membrane thickness (0.6-1.2 mm) to gain a detailed understanding of the oxygen transport properties in the fluorite/perovskite composites. The activation energy of oxygen permeation through the uncoated GDC/LSCF membrane closely corresponds with that of the oxygen ion conductivity of LSCF. On the contrary, the surface coating lowers the activation energy of the GDC/LSCF membrane to a level that is similar to that of the oxygen ion conductivity of GDC. This observation indicates that surface modification has an important role in enhancing the contribution of GDC to the GDC/LSCF (fluorite/perovskite) composite with respect to oxygen transport. In addition, the impact of the surface exchange kinetics on the oxygen transport of surface-coated membrane was also investigated. The calculated characteristic thicknesses (below which oxygen permeation is controlled by surface exchange kinetics) of the surface-coated GDC/LSCF membrane are 0.33 mm and 0.36 mm at 950 degrees C and 900 degrees C, respectively. Accordingly, the oxygen transport of the surface-coated GDC/LSCF membrane with thickness of about 1 mm is governed by not only oxygen ion diffusion but also the surface exchange kinetics. Reliable oxygen ion conductivity of the GDC/LSCF composite has been verified from oxygen permeation flux data using the modified Wagner equation. (C) 2013 Elsevier B.V. All rights reserved.