The expulsion of hydrocarbons from kerogen is the initial step in the primary migration process, during which the composition of the expelled petroleum is enriched in saturated and aromatic hydrocarbons while the retained bitumen is enriched in polar compounds. The physical and chemical principles responsible for this chemical fractionation are not well understood, and numerous theories have been proposed to explain the expulsion process. A multicomponent equilibrium likely exists between the kerogen matrix and the expelled fluid during petroleum generation and expulsion. To test whether such equilibrium can explain the nature and extent of chemical fractionation, an extended Flory-Rehner Regular Solution Theory model was developed and applied to a series of kerogens of varying structure, generative potential, and maturity. Thermodynamic parameters for immature Type II and IIIC kerogens (solubility parameter, cross-link density, and native swelling) were derived experimentally and extended to higher maturity. Mixtures of model compounds with well-defined properties were created to reflect the composition of primary generated products of kerogen thermal decomposition. Multicomponent equilibrium then was calculated under closed system conditions. The amount and composition of modeled expelled products are most sensitive to the generative potential and cross-link density of the kerogen. In general, lower source richness and cross-link density is associated with bitumen retention and a relative enrichment of the aliphatic components in the expelled fluid. Higher source richness and cross-link density result in earlier expulsion of fluids that are enriched in polar components. The predicted compositions of expelled fluids correspond well with the compositional range observed for produced petroleum. The predicted bitumen (kerogen-retained, soluble organic matter) compositions are uniformly > 50% C-14+ NSOs at all levels of maturity for all modeled kerogens. A chemically driven equilibrium mechanism based on Regular Solution Theory can explain almost completely the nature and extent of chemical fractionation that takes place during expulsion.