Silica-supported bismuth molybdate catalysts have been prepared by impregnation, structurally characterized and examined as improved catalysts for the selective oxidation of propylene to acrolein. Catalysts with a wide range of loadings (from 10 to 90 wt%) of beta bismuth molybdate (b-Bi 2 Mo 2 O 9 ) were studied to provide a better understanding about the distribution of active sites, and to elucidate the role of lattice oxygen in the reaction. The catalyst containing 50 wt% of beta bismuth molybdate on SiO 2 was found to possess good distribution of active sites and sufficiently high lattice oxygen, which resulted in an extraordinary increase of the catalytic activity.