DURING investigations of gas phase heterogeneous catalysis it is often necessary to demonstrate that the reaction products arise from the heterogeneous catalytic reaction and not from a homogeneous reaction. A commonly used method is to compare results obtained with the reactor charged with catalyst with those obtained with the reactor empty. If no reaction occurs in the empty reactor experiment it is concluded that the reaction being studied is catalytic. This conclusion is not always justified, however, since combined homogeneous-heterogeneous reactions may occur1-3. The products of the catalytic reaction may themselves undergo further homogeneous reaction with the formation of new products. A more subtle situation is one where the product of the heterogeneous catalytic reaction in turn initiates a homogeneous reaction of the reactants. The work recorded here on the oxidation of propene is an example of the latter case.