Abstract: In a hydrocracking process a feed mixture comprising: heavy oil containing asphaltenes and sulfur moieties; an oil-soluble, metal-containing compound additive (such as iron pentacarbonyl or molybdenum 2-ethyl hexanoate), which additive is operative to impede coalescence of coke precursors and which forms hydrocracking catalytic particles in situ; and, optionally, a hydrocarbon diluent which is a solvent for asphaltenes and which will assist with dispersion of the additive; is mixed for a prolonged period at low temperature (e.g., 80.degree. C.-190.degree. C.) in a first vessel or vessels to disperse the additive without significantly decomposing the additive. Preferably, the product mixture is then digested in a second vessel or vessels by mixing it at an elevated temperature (e.g., 250.degree. C.), to decompose the additive. The resulting mixture is then heated to hydrocracking temperature (e.g., 450.degree. C.) and introduced into a reactor.