The formation of coke from asphaltenes during thermal cracking is significantly affected by both reactions with the liquid components and the solvent properties of the liquid medium. The roles of both solubility phenomena and chemical reactions were studied by reacting Athabasca asphaltenes in a closed batch reactor at 430 degreesC. Reactions of asphaltene in maltene at different concentrations and reactions in a series of aromatic solvents (1-methyl naphthalene, naphthalene, and tetralin) were used to investigate the role of solvent properties and hydrogen donation reactions. The most important characteristics of the liquid phase were hydrogen donating ability and the ability to initiate cracking reactions. The latter mechanism was confirmed by adding n-dodecyl sulfide as an initiator compound. A modified kinetic model for coke formation, incorporating phase separation and hydrogen transfer to the asphaltenes, was consistent with the experimental results over a range of asphaltene concentrations and solvent conditions.