Ultrafine mixed Fe-Mo oxide particles are prepared by the sol-gel technique using citric acid as a complex agent. The formation process of the ultrafine oxides is studied by using XRD, DTA-TG, FT-IR, TEM, and BET surface area measurement, It is found that the morphology and structure of the oxide particles are significantly dependent on the process parameters such as thermal treatment temperature (T), pH of the starting solution, and the molar ratio of citric acid to metallic ions (L/M), The formation temperature of crystalline Fe-2(MoO4)(3) by the sol-gel process is much lower than that by the coprecipitation method. The catalytic properties of ultrafine Fe-Mo oxide particles are tested and compared with those of large oxide particles having the same composition. It is shown that for selective oxidation of toluene to benzaldehyde the ultrafine Fe-Mo oxide particles exhibit higher benzaldehyde selectivity and toluene conversion than the large oxide particles, The unique catalytic properties of ultrafine Fe-Mo oxide particles may be correlated to the higher mobility of lattice oxygen species in the ultrafine oxide particles and their higher BET surface area.