A process for converting heavy hydrocarbons into light hydrocarbons which comprises contacting, in a reaction zone, a heavy hydrocarbon having an API gravity at 25.degree. C. of less than about 20, such as Boscan heavy crude oil or tar sand bitumen, with a liquid comprising water and with an effective amount of selected catalyst materials such as iron (II and/or III) oxides, sulfides or sulfates, in the absence of externally added hydrogen, at a temperature between greater than about 340.degree. and about 480.degree. C. and at a pressure between about 1350 kPa (about 196 psig, about 13.2 atm) and about 15,000 kPa (about 2175 psig, about 148 atm), for a time sufficient to produce a residue and a vapor phase comprising light hydrocarbons, gaseous product 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.