An innovating coupling between the deoxygenation of benzoic acid and the oxidation of propylene was set up and gave new information about the mechanism involved in the oxidation of propylene on a Co-Mo based oxide catalyst. The production of CO, during the catalytic reaction is bound to the formation of benzene and benzaldehyde. The first case corresponds to the removal of the carboxyl function of the benzoic acid. The second case is the evidence that benzaldehyde and products coining from the oxidation of propylene are formed on the same catalytic sites during the Mars, and van Krevelen cycle. In this cycle, the oxygen atoms used for the oxidation come from the benzoic acid by the intermediate of the oxide lattice. In particular, it has been demonstrated that the oxidation of propylene involves lattice oxygen atoms far from each other in such a way that the reaction leaves single oxygen vacancies at the surface of the catalyst. Such coupling between a deoxygenation reaction and an oxidation one is reported for the first time. (c) 2005 Elsevier B.V. All rights reserved.