Silica was found to promote the photooxidation of propene by molecular oxygen, yielding acetaldehyde, propylene oxide, and propionaldehyde. The yield of and selectivity to propylene oxide were improved significantly by modification of silica with 1 wt% magnesium oxide loading, The product selectivity was affected by the amount of magnesium and the preparation method, Mg K-edge XANES and phosphorescence spectra indicated that the active sites on the Mg-loaded silica for epoxidation of propene are highly dispersed and isolated magnesium oxide species. The photoactive sites on bare silica are briefly discussed.