Heavy liquid hydrocarbon oil, such as petroleum derived tars, predominantly boiling over 425.degree. C., are upgraded to products boiling below 425.degree. C., without substantial formation of insoluble char, by heating the heavy oil with hydrogen and a hydrogen transfer solvent in the absence of hydrogenation catalyst at temperatures of about 320.degree. C. to 500.degree. C., and a pressure of 20 to 180 bar for 3 to 30 minutes. The hydrogen transfer solvents are polycyclic compounds free of carbonyl groups, e.g., pyrene, and have a polarographic reduction potential which is less negative than phenanthrene and equal to or more negative than azapyrene.