In a process for converting heavy hydrocarbons, such as petroleum or shale oil, into light hydrocarbons by distilling under the pressure of an inert gas, the operation is conducted at a temperature not exceeding 410/sup 0/C and under an accurately regulated pressure, the gas being circulated through the still and the condenser by means of a pump. The oil in the still may be agitated by stirring vanes or by blowing the gas through it. Hydrogen, nitrogen, carbon dioxide, methane, or gases generated in the distillation may be used as pressure media; the gas is heated before its admission to the still. A pressure of from 11 to 12 atmospheres is used in treating gas oil. Specification 10,277/89 is referred to.