The structure and catalytic properties of ultrafine Mo-based oxide particles prepared by the sol-gel technique are studied by using XRD, TEM, TPR, FT-Raman, BET surface area measurement, and microreactor tests. It is shown that for partial oxidation of toluene to benzaldehyde, the ultrafine Fe-2(MoO4)(3) and Ce-2(MoO4)(3) particles exhibit higher benzaldehyde selectivity and toluene conversion than the corresponding larger oxide particles prepared by the conventional coprecipitation method. The unique catalytic properties of ultrafine Fe-2(MoO4)(3) and Ce-2(MoO4)(3) particles may be correlated to the higher mobility of lattice oxygen ions in the oxides and their higher BET surface area. For the ultrafine Mo-based oxides prepared by the sol-gel process, the selectivity to benzaldehyde follows the order Ce-2(MoO4)(3) > Fe-2(MoO4)(3) > La-2(MoO4)(3) > MoO3, which is in good agreement with the reducibility of Mo ions and reactivity of lattice oxygen ions in the ultrafine Mo-based oxides. These results reveal that the lattice oxygen ions in the Mo-based oxides are the main active species for partial oxidation of toluene to benzaldehyde. The results of in situ LRS studies indicate that the terminal Mo=O bonds in Ce-2(MoO4)(3) are the main reactive species for partial oxidation of toluene to benzaldehyde.