Abstract: A heavy hydrocarbon oil, a substantial portion of which boils above 524.degree. C., is subjected to hydrocracking with a fractionated heavy oil recycle stream containing active additive particles. In the process, a slurry feed of (1) fresh heavy hydrocarbon oil feedstock and a heavy hydrocarbon recycle and (2) from about 0.01-4% by weight (based on fresh feedstock) of iron sulphate additive particles having sizes less than 45 .mu.m, is passed upwardly through a confined vertical hydrocracking zone. A mixed effluent is removed from the top of the hydrocracking zone, which is then passed through a hot separator vessel. From the bottom of the separator is withdrawn a liquid heavy hydrocarbon stream comprising heavy hydrocarbons and particles of the iron sulphate additive converted mainly to an iron sulphide phase. This separated liquid heavy hydrocarbon stream is fractionated to obtain a heavy oil which boils above 450.degree. C., containing the additive particles.