A PROCESS FOR TREATING HEAVY HYDROCARBON OIL WITH HYDROGEN GAS AT HIGH PRESSURES AND TEMPERATURES IN THE PRESENCE OF PARTICULATE CATALYSTS IS DISCLOSED HEREIN. THE PROCESS COMPRISES FEEDING THE HEAVEY HYDROCARBON OIL, PREFERABLY A VACUUM OR ATMOSPHERIC RESIDUUM, HYDROGEN GAS AND FINE PARTICULATE CATALYST IN THE SIZE RANGE OF UP TO ABOUT 600 MICRONS UPWARDLY THROUGH AN EXPANDED BED OF COARSE PARTICULATE CATALYST RANGING INSIZE FROM ABOUT 1/33 TO ABOUT 3/8 INCH AT A SUFFICIENT VELOCITY TO MAINTAIN THE BED IN AN EXPANDED, RANDOM MOTION STATE, REDUCING THE UPWARD VELOCITY OF THE HYDROCARBON OIL, HYDROGEN GAS AND FINE PARTICULATE CATALYST ABOVE THE EXPANDED COARSE CATALYST BED TO A VELOCITY SUFFICIENT TO ESTABLISH AN EXPANDED FINE CATALYST BED, AND WITHDRAWING THE TREATED HYDROCARBON OIL AND EFFLUENT GASES FROM THE HIGH PRESSURE, HIGH TEMPERATURE ZONE ABOVE THE FINE CATALYST BED.