NiMoS catalysts supported on MgO-Al2O3 oxides, with 95 and 80 mol% of MgO, were synthesized by sol-gel method. In order to study the Ni promoter effect, MgO-Al2O3 supports were impregnated with a pH = 9 solution of Mo and Ni-Mo, respectively; the catalysts were dried (D) and calcinated (C). Catalytic tests showed a Ni promoter effect of 4.5 on the NiMoMg95Al5-D catalyst and 8.5 on the calcinated one. The latter catalyst is more active than a commercial NiMo/Al2O3 catalyst. On the other side, the catalyst supported on Mg80Al20 solid did not show any Ni promoter effect. Raman and UV-vis diffuse reflectance spectroscopy showed that during the impregnation step, a strong support interaction with the ion MoO42- takes place on the Mo/MgO-Al2O3 solids. After calcination, MoO42- ion remained on the catalyst surface, but increased its interaction with the support. The presence of Ni-Th(2+), Ni-Oh(2+) and MoO42- ions on dried NiMo/Mg95Al5 catalysts was confirmed, as well as the presence of Ni-Th(2+), Ni-Oh(2+), MoO42- and Mo7O246- ions on the calcinated catalyst. This suggests that Ni2+ ion allows polymerization of MoO42- to Mo7O246-, produced by Ni-Oh(2+)-MoO42- and Ni-Oh(2+)-Mo7O246- Close interactions. The NiMo/Mg80Al20 solids also showed MoO3 species and a high Ni-Th(2+) concentration. Thus, the Ni promoter effect and therefore, catalytic activity decreased, due to the formation of Ni2+Th-MgO and Ni-Th(2+)-Al2O3 spinels. (C) 2009 Elsevier B.V. All rights reserved.