A process for converting a heavy hydrocarbonaceous chargestock to lower boiling products which process comprises reacting the chargestock with a catalyst concentrate in the presence of hydrogen, at hydroconversion conditions, said catalyst concentrate having been prepared by the steps comprising: (a) forming a precursor catalyst concentrate by mixing together: (i) a hydrocarbonaceous oil comprising constituents boiling above about 1050.degree. F.; (ii) a metal compound, said metal being selected from the group consisting of Groups II, III, IV, V, VIB, VIIB, and VIII of the Periodic Table of the Elements, in an amount to provide from about 0.2 to 2 wt. % metal, based on said hydrocarbonaceous oil; (b) heating the precursor concentrate to an effective temperature to produce a catalyst concentrate, wherein elemental sulfur is used an a sulfiding agent in an amount such that the atomic ratio of sulfur to metal is from about 1/1 to 8/1.