Though a variety of examples, it is shown that the catalytic oxidation reactions which operate via a Mars and van Krevelen mechanism imply ensembles of atoms containing variable valence cations and Lewis bases such as O2-, OH- Or PO43- able to act in the different elementary steps. The structure and the size of these ensembles, defined as ’inorganic oxide molecular clusters’, determine their reactional specificity. The examples chosen show that the ’designs’ of such ensembles are strongly dependent on the nature of the material, on their morphology (structure sensitivity as for MoO3, vanadyl phosphates), and on their structure (vanadyl phosphates, iron phosphates). It is also possible to build such ensembles upon dispersion of an active oxide on an inert support (MoO3/SiO2, V2O5/TiO2).