Halogenation of bitumen and asphaltenes was investigated to convert asphaltenes into maltenes. Halogenation resulted in products with increased penetration hardness. Three hypotheses were forwarded to explain these observations: stronger stacking as a result of electrostatic differences, free radical addition reactions, and increased hydrogen bonding. A model compound investigation was performed. No evidence was found that stacking,. strength increased as a result of electrostatic differences between halogenated and non-halogenated materials. Addition reactions were found on heating halogenated materials but not on mixing without heating. Evidence was presented of increased hydrogen bonding, although not necessarily stronger hydrogen bonding. Changes in the properties of mixtures of halogenated and non-halogenated materials were observed and, in one instance, the development of a new crystal configuration. On the basis of this work, the most plausible explanation for the increased hardness after halogenation bitumen and asphaltenes is an increase in hydrogen bonding.