In the Ni-Mo-O system, the addition of molybdenum oxide to nickel molybdate significantly increases its performance as a catalyst in the oxidative dehydrogenation of propane to propene, The most effective composition is Mo/Ni = 1.27/1, for which a selectivity of 63 mol% in propene is obtained at a propane conversion of 22 mol% (500 degrees C, s = 3.8 s, C-3/O-2/H2O/N-2 = 20/10/30/40). Several methods of preparation have been used and Mo/Ni ratios were varied from 0.90 to 2.15. Chemical analyses, X-ray diffraction patterns and infrared spectra show that the solid precursor of Mo/Ni > 1 catalysts contains two ammonium salts, NH4(NiMoO4)(2)OH . H2O and (NH4)(4)NiH6Mo6O24. 5H(2)O. During calcination these salts give rise to alpha-NiMoO4 and to a mixture of alpha-NiMoO4 and MoO3 (molar ratio NiMoO4/MoO3 = 1/5), respectively DTA/TGA shows that the relative rates of their decomposition during calcination depend on the method of preparation. These experiments pet-mit the precursors to be classified as type I, II, or III materials. The crystallization of MoO3 proceeds at a lower temperature for type I than for type II material (280 instead of 380 degrees C) and before the crystallization of alpha-NiMoO4 (ca 450-455 degrees C). No DTA or TGA signal accounts for crystallization of MoO3 or alpha-NiMoO4 in type III material. In calcined type I material, the polymorphic transition alpha --> beta-NiMoO4 is advanced because of the presence of MoO3, and MoO3 itself does not sublime easily, Type I catalysts exhibit better catalytic properties than other types. In differential conditions (500 degrees C, tau = 0.2 s), a synergetic effect is observed with Mo/Ni = 1.27 (type I) catalyst, the conversion of propane being maximum, Coherent interfaces between the (010) plane of alpha-NiMoO4 and the (100) plane of MoO3 are shown by transmission electron microscopy. As tentatively explained in ?he discussion, these interfaces are formed during calcination of type I precursors, the decomposition of which determines the way the reactive microdomains of NiMoO4 are distributed throughout the catalyst in the presence of, and/or onto, crystallites of MoO3. In turn, the catalytic properties of NiMoO4/MoO3 (Mo/Ni > 1) are enhanced for the oxidative dehydrogenation of propane to propene.