The regularities of formation of suspensions of dispersed molybdenum-containing catalyst nanosized particles synthesized in situ in heavy petrochemical feedstock with various compositions are studied depending on hydroconversion conditions. Ammonium heptamolybdate is used as a precursor. The following parameters are varied in the experiments: the precursor catalyst concentration, within 0-1% molybdenum; feedstock composition, both group and fractional composition; sulfur concentration in the feedstock, in the range of 0.87-3.39%; and temperature, in the range of 420-450 degrees C. It is found that the fraction of molybdenum sulfide in the composition of the catalyst grows and the concentration of coke in the hydroconversion residue insoluble in toluene decreases with increasing partial pressure of hydrogen sulfide in the reaction gas. Molybdenum compounds in the catalyst slightly affect the conversion of the fraction >500 degrees C but substantially decrease the yield of condensation products. The rate constants and activation energy of the desulfurization reaction of vacuum residue in the presence of the dispersed catalyst are calculated.