Abstract: Process for thermal cracking of residual hydrocarbon oils by(1) feeding the oil and a synthesis gas to a thermal cracking zone, the gas having a temperature sufficiently high to maintain the temperature in the cracking zone by direct heat exchange at 420.degree.-645.degree. C.;(2) separating the cracked products into (a) a gas, (b) at least one distillate fraction and (c) a cracked residue;(3) separating the cracked residue into a heavy hydrocarbon oil poor in asphaltenes and a heavy hydrocarbon oil rich in asphaltenes;(4) gasifying the oil rich in asphaltenes in the presence of oxygen and steam to produce synthesis gas; and(5) recycling synthesis gas from step 4 as feed to step 1.