The chemistry of preparation of molybdates, of TiO2 supported vanadium oxides and of V-P mixed oxides has been examined and discussed. The main factors which determine the nature of the active species are : (a) in the case of preparation of molydbates, the nature of molybdenum species in solution and the solid state reaction which free oxides; (b) in the case of preparation of supported vanadium oxide, the wetting by vanadium species of the support surface during the calcination and the reduction occurring at the vanadium sites during the reaction; and (c) in the case of preparation of V-P mixed oxide, the defective nature of the precursor and its transformation which occurs during the activation stage and under reaction conditions.