Thermal processes can be used to produce fuel oils and bitumens or can be applied to recover oil from shales. Thermal residues and bitumens (visbreaker bitumens) are normally less stable than straight run products and contain more asphaltene. This paper proposes a new way to stabilize thermal residues and bitumens by chemical treatment. The process is based on the fact that thermal residues contain chemical species with unpaired electrons. They are reacted with small percentages of phosphoric acid (from 0.2 to 5 wt %) in order to produce a carbonaceous insoluble matter which is settled or centrifuged. The stabilized product has a better resistance to aging and may contain less asphaltene than the original sample. Phosphoric acid is recovered from the insoluble matter and recycled to the process. Once free from the entrapped oil and from phosphorus, the insoluble residue ranges from 1 to 16 wt % of the original bitumen, depending on the amount of phosphoric acid added and On the quality of the bitumen sample. A "light" stabilizing treatment or a "heavy" deasphaltening process can be applied. The insoluble residue can be used as solid fuel, a component of coal-water emulsions, carbon black, or active carbon.