A random process for the selection of molecular fragments was used to build molecular distributions of different fractions from crude oils and related materials. By use of these molecular distributions and an additive group method, it was possible to theoretically build the solubility parameter distributions of the fractions. For asphaltene fractions, theoretical results show characteristic patterns that depend on the origin of the fraction. Such patterns are related to the stability behavior of the material, in agreement with experimental evidence. Also, it was found that solubility behavior can be related directly to the H/C ratio, while the heteroatom content shows very little influence on the solubility parameter distributions.