Abstract: This invention relates to a method for sulfiding hydrocarbon conversion catalysts, e.g., hydrotreating catalysts comprising at least one metal sulfide and passivating said sulfided catalyst. The catalysts which may be treated by the method of the present invention comprise an alumina or an silica-alumina support e.g., a zeolite, and at least one Group VI metal sulfide and/or at least one Group VIII metal sulfide. In particular, the present invention provides a process for continuously activating a fresh or regenerated catalyst comprising at least one Group VI or Group VIII metal oxide supported on a particulate refractory oxide support material by converting substantially all of said Group VI or Group VIII metal oxide to the corresponding metal sulfide and passivating the resulting activated catalyst.