Structural changes of the ammonium phospho-molybdic Wells-Dawson salt (NH4)(6)P2Mo18O62 were examined by several solid state techniques during the oxygen-assisted catalytic reaction of 2-butanol. A significant evolution of the catalytic behavior occurs while the working catalyst is in the course of its reorganisation to a Keggin structure. Precisely, a particular species formed in situ and presenting intermediate characteristics between those of Wells-Dawson and Keggin structures exerts a beneficial effect on the selectivity to methyl-ethyl-ketone. This high selectivity towards the oxidative dehydrogenation was however found to be ephemeral and the selectivity decline was correlated to a no proper stabilization of this most active intermediate. (C) 2008 Elsevier B.V. All rights reserved.