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.degree. C. of greater than about 20 and substantially free of vanadium and nickel values, i.e., less than 50 ppm, preferably less than 30 ppm, a gaseous product, and a residue is disclosed.