A treated decomposable compound of molybdenum, which has been prepared by the catalytic hydrogenation of a decomposable compound of molybdenum, wherein the molybdenum has a valence state greater than zero, or by the treating of the decomposable compound of molybdenum with a reducing agent, is mixed with a hydrocarbon-containing feed stream. The hydrocarbon-containing feed stream containing such treated decomposable compound of molybdenum is then contacted with a catalyst composition comprising a support selected from the group consisting of alumina, silica and silica alumina and a promoter comprising at least one metal selected from the group consisting of Group VIB, Group VIIB, and Group VIII of the Periodic Table to reduce the concentration of metals, sulfur, nitrogen, Ramsbottom carbon residue and/or heavies contained in the hydrocarbon-containing feed stream.