The amount of acidic sites of a series of SnO 2MoO 3 binary oxides was determined by n-butylamine titration and correlated to the catalytic activity to form acetone in the oxidation of propylene. It was found that the catalytic activity increases linearly with the surface concentration of acidic sites which are formed by the combination of stannic oxide and molybdenum trioxide. The same relation holds for other transition metal oxides combined with molybdenum trioxide which are also active for acetone formation. The role of the acidic sites is to promote the hydration of the olefin which is the first step of acetone formation.