Abstract: A process for converting heavy hydrocarbons into light hydrocarbons which comprises contacting, in a first zone, a heavy hydrocarbon having an API gravity at 25.degree. C. of less than about 20, such as Boscan heavy crude oil and tar sand bitumen, with a liquid comprising water, in the absence of externally added catalyst and hydrogen, while maintaining the first zone at a temperature between about 380.degree. and about 480.degree. C. and at a pressure between about 5000 kPa (about 725 psig, about 49 atm) and about 15,000 kPa (about 2175 psig, about 148 atm), for a time sufficient to produce a uniform reaction mixture; forwarding the uniform reaction mixture to a second zone wherein the temperature and pressure conditions of the first zone are maintained for a time sufficient to separate the uniform mixture into a residue and a phase comprising light hydrocarbons, gas and water, withdrawing the residue and said phase from the second zone; and recovering a light hydrocarbon product having an API gravity at 25.