The catalytic oxidation of methane was examined over 5% (w/w) NiO- on CeO2 or ZrO2. The particle size of single components was varied independently to investigate the effect of metal and support particle size on activity for complete methane oxidation. Catalyst activity is most affected by NiO particle size, however the support particle size also plays an active role. Results show that the particle size of ceria has a greater effect on activity than that of zirconia. This is attributed to ceria's greater oxygen transport capacity and contribution to the reaction mechanism. Changes in ceria particle size significantly affected the Arrhenius parameters, suggesting ceria forms part of the active site on a ceria-supported NiO catalyst. In contrast, zirconia particle size did not result in changes in activation energy, but affected only the number of catalytic sites as seen by changes in the pre-exponential factor. Ceria was also found to reduce deactivation of the catalyst due to increased oxygen transport to the active sites during reaction. (C) 2007 Elsevier B.V. All rights reserved.