Vanadium oxide is the main component of catalysts for selective oxidation reactions, in the form of dispersed oxide over supports, in multi-component mixed oxides, or incorporated inside inert or reactive matrixes. Some examples of the nature of the active site in V oxide-based catalysts and the effect of reaction conditions on its characteristics will be discussed: (a) the nature of the active layer in V/P/O, the catalyst for n-butane oxidation to maleic anhydride; (b) the role of V in V-polyoxometalates for liquid phase oxidations with oxygen, and (c) the nature of the active site in supported V oxide for propane oxidative dehydrogenation and o-xylene oxidation to phthalic anhydride.