This study examined the oxygen permeation behavior of Ce0.9Gd0.1O2-delta (Gadolinium-Doped Ceria, GDC)/Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) composite membranes fabricated using a conventional sintering technique. GDC/BSCF composite membranes with a relative density >95% could be obtained when a green compact of BSCF and GDC was sintered at 1150 degrees C for 5 h. It appears that GDC serves as a grain growth inhibitor because the average grain size of the composite decreased with increasing GDC content. The oxygen permeability of the BSCF and GDC/BSCF composite membranes strongly depends on the grain size and membrane thickness. The addition of GDC to BSCF resulted in a small grain size, low thermal expansion coefficient and high hardness. However, it is believed that oxygen permeation was blocked by GDC, as the oxygen permeability decreased with increasing GDC content. (C) 2010 Elsevier B.V. All rights reserved.