Asphaltenic material from Athabasca bitumen, with and without fine solids, was reacted at 430 degreesC under a nitrogen environment in the liquid phase, mixed with either 1-methyl naphthalene or maltene fractions from Athabasca. Phase behavior during coke formation was investigated by examining the coke produced. from selected reactions by scanning electron microscopy (SEM). Both fine solids and solvents were found to assist the dispersion of liquid coke spheres in an oil medium. Phase inversion, to an oil-in-coke structure, was observed in some sections of coke produced from pure asphaltenes. The thermodynamic feasibility of phase inversion was confirmed by calculating the entropy difference for a set of representative conditions. Qualitatively, as the volume fraction of coke increases, the probability of a phase inversion is increased. This result agrees with the behavior of polymer blends, where phase inversion occurs at high concentration.