Molybdenum manganese complex oxide was prepared and applied to the oxidation of toluene. The influences of various factors on the oxidation were observed and large amounts of benzyl acetate were formed in the reaction. When 30 mL water was added into the feed, the conversion of toluene and the maximum yield of benzaldehyde increased. Benzaldehyde was synthesized by the oxidation of toluene. The influences of various factors on the oxidation were observed and large amounts of benzyl acetate were formed in the reaction. To suppress the formation of benzyl acetate and increase the selectivity of benzaldehyde, water was added into the feed. It indicated that water not only increased the conversion of toluene, but improved the selectivity of benzaldehyde. The redox potential theory was used to explain the phenomenon. [GRAPHICS]