This invention relates to a process of catalytic hydroconversion of a heavy hydrocarbon oil containing a substantial portion of components having an atmospheric boiling point above 565 DEG C to give a product hydrocarbon oil containing components having a boiling point below about 565 DEG C. The process includes steps of mixing a heavy hydrocarbon oil with an oil soluble molybdenum compound, introducing the resulting mixture into a hydroconversion zone, introducing a reactor feed gas into the hydroconversion zone, and recovering the product hydrocarbon oil from the hydroconversion zone.