The deoxygenation of benzoic acid (DBA) is a probe reaction of the organisation of oxygen vacancies at the surface of molybdenum based oxide catalysts. In the present work, the DBA was used in coupling with the selective oxidation of propylene so that it could offer an insight into the nature of oxygen vacancies involved in the later reaction. By correlating the superficial oxidation state of the catalytic elements measured by X-ray photoelectron spectroscopy with the information provided by the DBA probe reaction, it was possible to progress in the description of the atomic scale architecture of the oxidation sites at work. Sites bringing boosted performances in the oxidation of propylene are those which are slightly reduced thanks to the presence of single (isolated) oxygen vacancies. Conversely, twin oxygen vacancies do not participate in the oxidation reaction. (C) 2006 Elsevier B.V. All rights reserved.