An attenuated:association model previously developed to describe the aggregation if asphaltenes In solution is extended to a description of the phase behavior of bitumen and asphalts. Work reported in the literature using various experimental techniques shows that these materials have Complex phase structures, but essentially consist of two amorphous phases: one is predominantly asphaltenes and the other is largely maltenes. In the work reported here, these materials were first modeled as pseudo-two-component mixtures, where one component consists of a self-associating component that,:for the most :part, corresponds to asphaltenes, while the second component is a non-self-associating component that essentially consists of maltenes. It is shown that the critical value of the Flory chi interaction,parameter is significantly reduced in these mixtures, relative to asphaltene solutions,, and Calculated binodals (coexistence curves) are very broad and asymmetric, with a phase boundary that approaches the pure maltene composition limit. This indicates that, above a critical value of chi, bitumen and asphalts phaseseparate into an almost-pure maltene,fraction. However, the asphaltene-rich phase consists of an appreciable non-self-associating fraction that varies with temperature and overall bitumen composition. Ternary phase diagrams were also calculated by assuming that bitumen is a pseudo-three-component system that consists of a self-associating component, identified as asphaltenes, saturates, and aromatics plus resins (combined). These ternary mixtures are predicted to phase-separate into two phases: one consists of asp,haltenesiand aromatics plus resins, but with Only small amounts of saturates, while the second, phase consists of a mixture of saturates and aromatics plus resins with only trace amounts of asphaltenes. The calculations are consistent with experimental measurements Of the glass-transition temperatures of. these materials and microscopic observations of phase behavior.