Magnesia supported Ni-Mo sulfide catalysts were prepared by three methods : by impregnation from aqueous solutions, by precipitation-adsorption of sulfides in a water-toluene emulsion, and by impregnation from non-aqueous solutions using dimethylsulfoxide and methanol as solvents. They were tested in : (i) the single hydrodesulfurization of thiophene (HDS), and (ii) the parallel hydrodesulfurization of thiophene (HDS,PY) and the hydrodenitrogenation of pyridine (HDN). The reference Ni-Mo/alumina catalyst was the commercial sample Shell 324. The catalytic activity of samples prepared by aqueous impregnation and by precipitation-adsorption was low, activity for (HDS) and (HDN) of the best sample being only about 30 and 20%, respectively, of the activity of the Shell 324 catalyst at 320 degrees C. The chemical and textural instability of the high surface area magnesia in aqueous solutions was the limiting factor. Much higher activity was achieved by non-aqueous impregnation, activity of the best Ni-Mo/MgO sample prepared by this method for (HDS), (HDS,PY) and (HDN) being 94, 290 and 105%, respectively, of the activity of the Shell 324 catalyst at 320 degrees C. As compared with the alumina supported catalyst, the special feature of the Ni-Mo/MgO catalysts irrespective of the preparation method was lower inhibition of HDS by parallel HDN and lower selectivity to HDN in parallel HDN and HDS.