The invention concerns a process for sulphurizing supported catalysts containing at least one element selected from group IIIB, including the lanthanides and actinides, group IVB, group VB and group VIB, said process being characterized in that said catalyst is pre-reduced with at least one reducing gas other than hydrogen before sulphurizing said catalyst. The invention also concerns the sulfide catalysts obtained by the process of the present invention as well as the use of the sulfide catalyst in a process for hydrocracking and hydrotreatment of hydrocarbon-containing feeds.