The hydroconversion of heavy oil in the presence of an in situ synthesized catalyst (MoS2) is studied. It is shown that ultrafine aggregates of amorphous components and crystalline particles of molybdenum sulfide with a particle diameter of 20-70 nm are formed from catalyst precursor (ammonium paramolydbate) emulsion under hydroconversion conditions in a medium of heavy oil. The kinetic characteristics of the hydroconversion of heavy oil and separate groups of its components-paraffin-naphthene hydrocarbons, aromatic hydrocarbons, resins, and asphaltenes-are determined.