The most common preparation of high surface area MgO (100-500 m(2) g(-1)) is calcination of Mg(OH)(2) obtained either by precipitation or MgO hydration or sol-gel method. Preparation of MoO3/MgO catalyst is complicated by the high reactivity of MgO to H2O and MoO3. During conventional aqueous impregnation, MgO is transformed to Mg(OH)(2), and well soluble MgMoO4 is easily formed. Alternative methods, that do not impair the starting MgO so strongly, are non-aqueous slurry impregnation and thermal spreading of MoO3. Mo species of MoO3/MgO catalyst are dissolved as MgMoO4 during deposition of Co(Ni) by conventional aqueous impregnation. This can be avoided by using non-aqueous impregnation. Co(Ni)Mo/MgO catalysts must be calcined only at low temperature because Co(Ni)O and MgO easily form a solid solution. Literature data on hydrodesulfurization (HDS) activity of MgO-supported catalysts are often contradictory and do not reproduced well. However, some results suggest that very highly active HDS sites can be obtained using this support. Co(Ni)Mo/MgO catalysts prepared by non-aqueous impregnation and calcined at low temperature exhibited strong synergism in HDS activity. Co(Ni)Mo/MgO catalysts are much less deactivated by coking than their Al2O3-supported counterparts. Hydrodenitrogenation (HDN) activity of Mo/MgO catalyst is similar to the activity of Mo/Al2O3. However, the promotion effect of Co(Ni) in HDN on Co(Ni)Mo/MgO is lower than that on Co(Ni)Mo/Al2O3. (C) 2003 Elsevier B.V. All rights reserved.