In the search of selective hydrodesulfurization catalysts with low hydrogenating function, Mo and NiMo catalysts supported on Mg-Al mixed oxides with molar ratios x=MgO/(MgO+Al2O3)=0.0, 0.05, 0.25, 0.50, 0.75 and 1.0 were prepared by the sol-gel method. The synthesized solids were characterized by surface area, TGA, X-ray diffraction, temperature programmed reduction (TPR), DRS, FTIR and FT-Raman spectroscopy and high resolution electron microscopy (HREM). The catalysts were tested in the thiophene hydrodesulfurization reaction. The results indicate that even with the incorporation of small amounts of magnesia to the alumina support, the hydrogenation function of the catalyst is substantially reduced. The changes in the hydrodesulfurization and hydrogenation functions of the catalysts seem to be due to the formation of magnesium-molybdate and also due to the formation of a NiO-MgO solid solution which takes away the promoting effect of Ni. Therefore, in NiMo catalysts supported on magnesia containing supports, it is necessary to use Ni/(Ni+Mo) atomic ratios greater than 0.3 to recover the promotion effect lost by the interaction of NiO with magnesia. The results also indicate that only the catalysts with low magnesia content (x<0.25) show textural stability.