The state and reactivity of lattice oxygen ions in the ultrafine Ce-Mo oxide particles prepared by the sol-gel method are studied by using X-ray diffraction, laser Raman spectroscopy, temperature-programmed reduction, X-ray photoelectron spectroscopy and pulse microreactor tests. It is found that the lattice oxygen ions in the ultrafine Ce-Mo oxides are the main active species for partial oxidation of toluene to benzaldehyde. By decreasing the size of complex Ce-Mo oxide particles to nanometric scale, the reactivity of lattice oxygen ions can be remarkably improved.