Rheological and X-ray scattering techniques were applied to investigate how the addition of phosphorus compounds to a refinery vacuum residue (Safaniya) modifies its macroscopic behavior. The modified vacuum residue has a colloidal structure different from the original product. Rheological data show the more "gellike" nature of the modified sample in the temperature range from 0 to 40 degrees C. Its higher polydispersity and lower fractal dimension D-f also indicate that a shift toward a more gellike state occurs. The modified sample flow properties appear to be strongly dependent on the chemical and structural modifications of the single separated fractions. Compared to the strong modifications involving the asphaltene fraction, the changes which occur to the resin fraction properties are less important.