A three step process is proposed for stabilizing thermal residues and bitumens, such as visbreaker residues, and for upgrading SR bitumens used to produce stable polymer-modified bitumens. The process is based on the addition of phosphorus compounds such as phosphoric or polyphosphoric acid to VB residues and bitumens, and to the utilization of the insoluble by-product for upgrading SR bitumens or for the preparation of very stable polymer modified bitumens.