A process for the preparation of novel highly active, highly selective hydrotreating catalysts. These catalysts are prepared, in bulk or in supported form, by contacting together and decomposing, in the presence of hydrogen, hydrocarbon, and sulfur, a catalyst precursor characterized by the formula B.sub.x [Mo.sub.3 S.sub.z ] where B is an ammonium ion, polyammonium ion, tertiary or quaternary phosphonium ion, or a hydrocarbyl substituted ammonium ion, hydrocarbyl substituted polyammonium ion, or hydrocarbyl substituted tertiary or quaternary phosphonium ion, x is 1 where B is a divalent cationic moiety, or 2 where B is a monovalent cationic moiety, [Mo.sub.3 S.sub.z ] is a divalent anionic moiety wherein z is an integer ranging from about 10 to about 46. A preferred precursor catalyst species is Mo.sub.3 (S.sub.2).sub.6 S.