The effects of temperature and composition on the rheology of asphalt binders is of practical importance. The asphalt binder should be solid enough at higher temperatures to prevent the asphalt from flowing, and it should be fluid enough at lower temperatures to prevent the asphalt from cracking. In this work it is shown that there is a transition from a higher temperature Newtonian fluid phase to a lower temperature viscoelastic fluid phase at 45 degrees C in the vacuum residue of Arabian medium/heavy crude oil that is associated with the heptane insoluble asphaltenes. The asphaltene content must be greater than 15 wt % for this transition to occur. Solid properties increase by a factor of 3 beyond the phase boundary. The low-frequency dynamic viscosity increases by a factor of 5. The transition is first order with an enthalpy change on heating of 0.5 cal/g. The position of the phase boundary in a temperature-composition diagram can be predicted by calculating points at which the free volume of the asphaltenic colloidal particles in the Newtonian fluid state vanishes. According to small angle X-ray scattering the micro-structure of the viscoelastic phase consists of irregularly shaped clusters with a characteristic length of 500 Angstrom. There is a polydispersity of cluster sizes. The fractal dimension of the average cluster is 1.5. The clusters are themselves composed of smaller scattering centers with a characteristic length of 40 Angstrom. The time-temperature superposition principle applies in each microphase, but the shift factors are discontinuous at the phase boundary. However, only the relaxation times and density of Maxwell elements must be rescaled by constant factors at the Phase boundary.