A slurry hydroconversion process is provided in which a catalyst precursor concentrate comprising an aqueous solution of phosphomolybdic acid and a heavy oil is contacted with hot hydrogen to vaporize the water from the concentrate. The resulting catalyst precursor concentrate is introduced into a hydrocarbonaceous chargestock and the resulting mixture is heated in the presence of added hydrogen to convert the phosphomolybdic acid to a solid molybdenum-containing catalyst. The resulting slurry is subjected to hydroconversion conditions.