Ceramic systems of Bi2O3 and gadolinia-doped ceria (GDC) solid mixture were prepared as catalysts for direct methane oxidation. These systems were characterized by temperature-programmed reduction using hydrogen and carbon monoxide, temperature-programmed reaction of methane, fixed-temperature direct methane oxidation, and X-ray diffraction analysis. Adding Bi2O3 to GDC promotes both hydrogen and CO oxidation activities, because of the presence of surface Bi2O3 and the high content of mobile oxygen in Bi2O3. The reactivity of CO with surface lattice oxygen is enhanced to a higher extent than that of H-2, and this enhanced extent shows a maximum in Bi2O3 content. Such a maximum also exists for the catalytic activity of direct methane oxidation. A synergistic effect occurs due to a combination of the high methane reactivity of GDC and the high content of mobile oxygen in Bi2O3. The CO2 selectivity of direct methane oxidation can be modulated by varying the Bi2O3 content. The mixing of Bi2O3 with GDC also increases the self-de-coking capability of the catalyst during direct methane oxidation, which stabilizes the activity. (C) 2008 Elsevier B.V. All rights reserved.