Heavy oils containing metalloporphyrins principally of nickel and vanadium are demetallized using an oxidizing agent such as aqueous hydrogen peroxide and catalytic amounts of phosphoric acid, preferably with tungstic acid in combination with a phase transfer agent. Up to 99% of the Ni and V are deposited in the aqueous phase and are removed from the oil. The homogenous, water soluble reactants and catalyst have the advantage of being separated more easily from the Ni and V dissolved in the aqueous phase than the same metals deposited on solid phase heterogeneous catalysts.