The oxidation of n-butane to maleic anhydride is possible under fuel-rich conditions (nC(4)/O-2 = 0.6) at 400 degrees C by modifying the vanadium phosphorus oxide (V-P-O) catalysts by doping with Co or Mo; it is impossible on the undoped V-P-O catalyst, which deactivates under the same conditions. The doped precursors were prepared by reduction of the dihydrate VOPO4, 2H(2)O, and dopants were introduced at different moments of the preparation from cobalt acetylacetonate and ammonium heptamolybdate solutions, respectively. The Co-doped V-P-O catalyst (Co/V = 0.77%) conserves a higher capacity for reoxidation of the surface (higher V5+/N4+ ratio) and maintains a higher surface distribution of the V, P, and O atoms under fuel-rich conditions: Oz consumption is not total. Maleic anhydride is then produced and the catalysts do not deactivate. The Mo-doped V-P-O catalysts (Mo/V from 0.88% up to 2.56%) do not give performances as good as those for the Co-doped V-P-O catalyst. It appears that it is necessary to dope at a higher percentage, Mo/V = 2.56%, in order to achieve higher selectivity for MA. The corresponding catalyst is less active (5.3% nC(4) conversion instead of 25.5%) than the Co-doped V-P-O catalyst. This study demonstrates that the nature of the dopant is more important than its way of introduction into the V-P-O matrix. It opens new routes for adapting V-P-O catalysts for oxidizing n-butane to maleic anhydride under fuel-rich conditions and for developing new technologies.