HIGHER MOLECULAR WEIGHT HYDROCARBBONACEOUS MATERIALS CONTAINING POLYNUCLEAR SUBSTANCES, SUCH AS HEAVY PETROLEUM CRUDES AND HIGHER BOILING PETROLEUM FRACTIONS, INCLUDING RESIDUAL FRACTIONS, SHALE OIL, TAR SAND OIL, OIL AND TAR FROM COAL AND COAL ITSELF ARE HYDROCRACKD AT AN ELEVATED TEPERATURE RANGING FROM 250*C. TO 500*C. AND IN THE PRESENCE OF HYDROGEN AT A HPRESSURE OF 200 TO 3000 P.S.I.G., WHILE INTIMATELY MIXED WITH A HOMOGENEOUS CONTINUOUS LIQUID PHASE CATALYST SYSTEM COMPRISING PREDOMINATELY ONE OR MORE PHOSPHORIC ACIDS THERMALLY STABLE UNDER THE HYDROCRACKING CONDITIONS, SUCH AS, PYROPHOSPHORIC ACID AND POLYPHOSPHORIC ACIDS HAVING AN EMPIRICAL ACID AND H2O:P2O5 OF 3 OR LESS, ANDA MINOR PROPORTION, E.G., 10% BY WEIGHT OF CERTAIN METAL POLYHALIDES WHICH ARE ACTIVE HYDROCRACKING CATALYST, E.G., ESPECIALLY ZINC, GALLIUM AND ANTIMONY HALIDES.