A complete characterization of the solid-state behavior of the Wells-Dawson heteropolycompounds in relevant conditions for a catalytic oxidation reaction in the gas phase was obtained by following the evolution of the Wells-Dawson salt under reducing and oxidizing model conditions by in situ X-ray diffraction analyses and in situ Raman spectroscopy. It was found that the oxido-reduction strength of the working condition is a tool to control the rearrangement of the Wells-Dawson compound into oxide species. Precisely, an oxidizing feed was shown to stabilize the Wells-Dawson compound, i.e. delay the irreversible transformation of the structure to a mixture of oxides, and thus may be used to modulate the catalytic behavior of the catalyst. Looking for the influence of the oxido-reduction atmosphere, an operando setup combining Raman spectroscopy with online analysis of gaseous products of the catalytic propene oxidation was then used to gain further information on the solid-state behavior of Wells-Dawson heteropolycompounds at work. Along a reaction temperature profile, a peak of propene conversion could be obtained. This was hypothesized to be due to the in situ formation of a propene-containing bronze Mo oxide looking phase. The formation of such phase seems to be facilitated when a more reductive atmosphere is applied on the Wells-Dawson during the reaction. (C) 2009 Elsevier B.V. All rights reserved.