A simple attenuated association model for the aggregation of asphaltenes in solution is described. The model has only two parameters, a single equilibrium constant and the molecular weight of the "monomer" (unassociated asphaltene molecule). Both parameters were estimated from results reported in the literature and account for the variation of the degree of association with the concentration very well. Using Flory's treatment of reversible association and Prigogine's result that the chemical potential of an associating species is equal to the chemical potential of the monomeric species, equations for the free energy of mixing and its derivatives are derived. These equations were then used to model the phase behavior of solutions. Good agreement with observed experimental behavior was obtained using solubility parameters for asphaltenes in a narrow range near 20 MPa0.5, consistent with values reported in the literature.