The effect of gallium over CoMo/Al2O3 and NiMo/Al2O3 catalysts was investigated in the hydrodesulfurization of 4,6-dimethyldibenzothiophene. The gamma-Al2O3 support was modified, before the catalyst preparation, by impregnation with an aqueous solufion of Ga(NO3)(3).8H(2)O and calcined at 450 degrees C. The catalytic results were correlated to the catalysts physicochemical properties which were obtained by different characterization techniques such as X-ray diffraction, X-ray Photoelectron Spectroscopy, Transmission Electron Microscopy and Diffuse Reflectance Spectroscopy. A change of the promoter (Co2+ or Ni2+) interaction with the support is induced by the affinity of gallium (at low loadings) for the tetrahedral sites of alumina, leading to an increase in the octahedral species of Ni or Co. This improves the decoration of the MOS2 slabs. Gallium also modifies the interaction of MOS2 with the support, decreasing the numbers of Mo-O-Al anchors and increasing the stacking of the MOS2 slabs. These modifications improve the activity of the NiMo and CoMo catalysts in the deep HDS of 4,6-DMDBT. (C) 2008 Elsevier B.V. All rights reserved.