High surface area MoO3/MgO was prepared by the reaction of MgO (250-300 m(2) g(-1)) with a slurry of ammonium heptamolybdate in methanol or anhydrous ethanol at the alcohol boiling point. The low solubility of ammonium heptamolybdate was sufficient for its gradual transport to the support surface: molybdena species were deposited and ammonia was evolved. Catalytic activities in the hydrodesulphurization of benzothiophene of the MoO3/MgO samples were comparable to the activity of the reference commercial MoO3/Al2O3 catalyst.