The formation of polycondensation products in the hydroconversion of heavy oil feedstock is studied in the flow regime in the presence of an in situ synthesized ultrafine catalyst (MoS2). Experiments show that, to a conversion level specific to every feedstock, the coke yield attained by variation in temperature or contact time remains close to zero; subsequently it begins to grow exponentially. The observed regularity is associated with a difference in the rates of reactions involving feedstock components and the presence of coke precursor-asphaltenes-in the feedstock. A relationship is revealed between the amount of formed coke, the content of asphaltenes in the feedstock, and the Hildebrand solubility parameter, which characterizes the ability of the dispersion medium to maintain asphaltenes in the dispersed state without agglomeration.