The structure and properties of nanosized particles of a molybdenum-containing catalyst withdrawn from the reaction system with heavy hydroconversion products have been studied. The particle size of the catalyst and the features of its structure and phase composition have been determined. The particles of MoS2 synthesized in situ in the hydroconversion reactor have been shown to possess a layered structure. Individual slabs have a thickness of 0.6-0.7 nm and an average length of 9.8 nm. It has been found that monolayer particles aggregate to form stacks and spheres and are adsorbed on the surface of coke particles under conditions that disturb the equilibrium state of catalyst dispersion in the reaction medium. The distribution of the catalyst in heavy fractions of the reaction products has been studied. It has been shown that molybdenum is almost quantitatively transferred to the 500 degrees C+ fraction of the hydroconversion product, which is important for recycling the catalyst.