Nowadays, the environmental regulations that impose very low sulfur content in diesel cuts have spurred research in hydrotreating catalysts. The commercial hydrodesulphurization (HDS) catalysts consist of an active phase of molybdenum or tungsten (Mo, W) promoted by nickel (Ni) or cobalt (Co) usually supported on alumina. Nevertheless, it has been recognized since the very first studies related to these solids (CoMo, NiMo, NiW/Al2O3) that alumina is not an inert carrier. On one hand, the promoter ion, Co or Ni, can react with the support and occupy octahedral or tetrahedral sites in external layers or even form an inactive phase such as CoAl2O4 (NiAl2O4) depending on the preparation conditions. On the other hand, the molybdates (tunsgtates) interact strongly with the support from the first steps of catalysts preparation through Metal-O-Al anchors. It has suggested that the improvement of HDS catalysts is closely related to the formation of the so-called “NiMoS” (CoMoS, NiWS) phase. Thus, the amount of these phases should be increased by reducing the concentration of inactive species to improve the performance of HDS catalysts. Therefore, the effect of gallium over the CoMo/Al2O3 (CoMo) and NiMo/Al2O3 (NiMo) catalysts was investigated in the HDS of 4,6-dimethyldibenzothiophene. The γ-Al2O3 support was impregnated (before the catalysts were prepared) with an aqueous solution of Ga(NO3)3.8H2O and calcined at 450 ºC. The catalytic results were correlated to the catalysts physicochemical properties which were obtained by different characterization techniques such as X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM) and Diffuse Reflection Spectroscopy (DRS). A change of promoter (Co2+ or Ni2+) interaction with the support is provoked by the affinity of gallium (at low loadings) to the tetrahedral sites of alumina, inducing to an increase of the octahedral species of Ni or Co. This improves the decoration of the MoS2 (WS2) slabs. Gallium also modifies the interaction of MoS2 (WS2) with the support, decreasing the numbers of Metal-O-Al anchors and increasing the MoS2 slabs stacking. These modifications improve the activity of the NiMo, CoMo and NiW catalysts in the deep HDS (4,6-DMDBT HDS).