A process is disclosed 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 400.degree. and about 480.degree. C. and at a pressure at least about 690 kPa (about 100 psig, about 6.76 atm) and less than about 5,000 kPa (about 725 psig, about 148 atm), for a contact time under continuous flow conditions sufficient to produce a uniform (i.e., intimate) reaction mixture; forwarding the uniform reaction mixture to a second zone wherein the temperature and pressure conditions of the first zone are maintained at substantially steady state conditions 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 10 or 5 ppm, a gaseous product, and a residue.