We develop an analytical solution of the associative Ornstein-Zernike integral equation in the Percus-Yevick approximation to model asphaltene structural properties and phase separation in model oil dispersions as a function of pressure, temperature and composition. To account for association, asphaltenes are characterized as hard spheres with active association sites placed on its surface; these sites are capable of linking with other similar species. Solvent molecules are modeled as hard spheres flexible linear chains. There is a sticky attraction between asphaltene and solvent. Using a calculation scheme similar to branched polymerization, asphaltene precipitation and the structural properties of the mixture are studied at various degrees of aggregation. Results show how the occurrence of instability correlates with pair potential parameters, sizes, density and composition of the system. These results compare well with experimental evidences. (c) 2006 Published by Elsevier B.V.