(PPh₃)Mo(CO)5 and (PPh₃)₂Mo(CO)₄ were synthesized by the reaction of molybdenum hexacar-bonyl with triphenylphosphine and applied as precursors to hydrocracking of vacuum residue under high-pressure and high-temperature conditions. (PPh₃)₂Mo(CO)₄ could also be synthesized by the reaction of (PPh₃)Mo(CO)5 with triphenyl phosphine. A commercial precursor (Mo-octoate) for hydrocracking of vacuum residue was used for comparison. The thermal decomposition behavior of (PPh₃)Mo(CO)5, (PPh₃)₂Mo(CO)₄, and Mo-octoate was also examined by the thermogravimetric analysis. The TGA curve of (PPh₃)Mo(CO)5 and (PPh₃)₂Mo(CO)₄ showed a similar weight-loss pattern. (PPh₃)Mo(CO)5 and (PPh₃)₂Mo(CO)₄ were decomposed into Mo metal and ligands rapidly in the range of 140 °C~270 °C. There were no ligands bound to a metal center of (PPh₃)Mo(CO)5 and (PPh₃)₂Mo(CO)₄ at the reaction temperature (430 °C) of hydrocracking. The amount of coke formed after hydrocracking over (PPh₃)Mo(CO)5 and (PPh₃)₂Mo(CO)₄ was 2.3% and 0.5%, respectively. Upgrading the qualities of heavy oils is an important issue in the energy industry. It is not easy to achieve the complete conversion of vacuum residue due to coke forming during hydrocracking of vacuum residue. This study showed that (PPh₃)₂Mo(CO)₄ was considerably effective in reducing coke formation.