High-quality roofing asphalts are found to contain high-molecular-weight asphaltenes which exhibit a high tolerance for saturates and have a low carbon-to-hydrogen (C/H) ratio. These asphaltenes are believed to possess a few long, paraffinic side chains rather than many short chains or naphthenic groups. Evidence supporting this hypothesis was provided by the results of experiments in which highly aromatic asphaltenes were alkylated with various saturates by exposure of asphaltene-saturate mixtures to the 3-Mev electron beam of a Van de Graaff accelerator. The base asphaltenes were alkylated with up to 30 per cent of their weight of saturate, producing alkylated asphaltenes of considerably reduced C/H ratio and of increased molecular weight. Using paraffins of carbon numbers 16 to 25, it is estimated that 6 to 10 side chains were present in the average alkylated asphaltene molecule. The blending characteristics were improved in that the saturate tolerance was greatly increased; in addition, coating-grade asphalts blended from the alkylated asphaltenes had improved durability. Decane was less effective as an alkylating agent than were the C16 to C25 paraffins, and waxes in the C40 range presented problems in separation of unreacted wax from the alkylated asphaltene product. Less improvement in durability was found with branched paraffins such as squalane and with naphthenic saturates, thus indicating the desirability of long chain length. Asphalt compositions having these improved charac eristics can be prepared by exposing either the whole asphalt or an asphaltene-rich fraction, together with suitable alkanes, to high-energy ionizing radiation at a dosage of about 109 rads.