Al2O3-SiO2 mixed oxide has been investigated as a support for hydrotreating catalyst with variation of its composition [Si/(Si + Al) = 0.06, 0.12, 0.31, 0.56, 0.78] and its interaction with the surface active metals (NiMo). The composition of support and surface species (NiMo) of catalysts were characterized by specific surface area, atomic absorption, SEM-EDX, XRD, temperature programmed reduction (TPR), Raman analysis, scanning electron microscopy (STEM) and transmission electron microscopy (TEM). Incorporation of SiO2 in Al2O3 promotes a weak interaction between the active phases and particularly catalyst that predominated with SiO2 content. The oxide and sulfided catalysts characterization indicated that the effect of support is responsible to form different catalytic sites. Crystallization of MoO3 phases and a relatively longer crystal of MoS2 in the sulfided catalyst were attributed to an increasing SiO2 content in the support. The catalytic behavior of the NiMo supported catalysts is explained in terms of structural changes on the surface due to the support and active metal interactions. The activity of the different catalysts evaluated in the thiophene hydrodesulfurization reaction was higher for the catalyst having lower SiO2 content in the support. (C) 2007 Elsevier B.V. All rights reserved.