A detailed study on the influence of the addition of molybdenum ions on the catalytic behaviour of a selective vanadium-magnesium mixed oxide catalyst in the oxidation of n-butane has been performed. The catalysts have been prepared by impregnation of a calcined V-Mg-O mixed oxides (23.8 wt% of V2O5) with an aqueous solution of ammonium heptamolybdate, and then calcined, and further characterised by several physico-chemical techniques, i.e. S-BET, XRD, FTIR, FT-Raman, XPS, H-2-TPR. MgMoO4, in addition to Mg3V2O8 and MgO, have been detected in all the Mo-doped samples. The incorporation of molybdenum modifies not only the number of V5+-species on the catalyst surface and the reducibility of selective sites but also the catalytic performance of V-Mg-O catalysts. The incorporation of MoO3 favours a selectivity and a yield to oxydehydrogenation products (especially butadiene) higher than undoped sample. In this way, the best catalyst was obtained with a Mo-loading of 17.3 wt% of MoO3 and a bulk Mo/V atomic ratio of 0.6. From the comparison between the catalytic properties and the catalyst characterisation of undoped and Mo-doped V-Mg-O catalysts, the nature of selective sites in the oxidative dehydrogenation of n-butane is also discussed.