Abstract: The hydroprocessing of heavy oils is improved by the use of a high activity slurry catalyst prepared by sulfiding an aqueous Group VIB metal compound with a gas containing hydrogen sulfide to a dosage greater than 8 SCF of hydrogen sulfide per pound of Group VIB metal. After introducing the slurry catalyst into the heavy oil, and subjecting the mixture to elevated temperatures and partial pressures of hydrogen, the mixture is treated in a fixed or ebullated bed of hydrodesulfurization/hydrodemetalation catalyst under hydroprocessing conditions.