Catalytic partial oxidation of methane to synthesis gas over ZrO2 and yttrium-stabilized zirconia (YSZ) is studied using O-2 and N2O as oxidants. ZrO2 is much more active than YSZ in oxidation of methane with N2O- In contrast, YSZ is significantly more active than ZrO2 when O-2 is used as an oxidant. The presence of O-2 does not influence the rate of N2O decomposition over ZrO2 and YSZ, while the presence of H2O in the system decreases N2O conversion significantly. O-2 and N2O are activated at different active sites. Y-induced oxygen vacancies are active for O-2 activation, whereas oxygen co-ordinatively unsaturated Zr cations (Zr-CUS) located at corners, edges, steps and kinks are responsible for N2O activation. These sites are also capable of dissociating H2O, resulting in competition between H2O and N2O. As compared with N2O, molecular O-2 is easier to be activated over YSZ and ZrO2. (c) 2005 Elsevier B.V. All rights reserved.