Mordenite catalysts incorporating CeO₂ in their cavities have been prepared by the hydrolysis of Ce³⁺ ions in the cavities with moist ammonia gas, following calcination at 550°C. The CeO₂ mordenite zeolite (CeO₂/MZ) is active for oxidation of p-xylene at 130°C with oxygen, but CeO₂ and mordenite are inactive even at 160°C. X-Ray photoelectron spectroscopy (XPS) analyses indicate that the concentration of Ce in the mordenite surface regions is nearly equal to that in the bulk. The amount of saturated nitrogen adsorbed and the effective pore distribution decrease in CeO₂/MZ has been compared with that for H⁺/MZ. The decreasing amount of adsorbed saturated nitrogen is nearly equal to the CeO₂ volume in CeO₂/MZ. Thus, the nitrogen adsorption isotherm for the CeO₂/MZ indicates a decrease in the effective pore size due to the incorporation of CeO₂ into the cavities. In addition, 1,3,5-trimethylbenzene is not oxidized at 160°C over CeO₂/MZ; the molecular size is larger than the diameter of the cavities. It has been concluded that CeO₂ is incorporated into the mordenite cavities, and that this incorporation accelerates oxidation.