The conformation of lowest energy of an asphaltene molecule of the Athabasca sand oil was calculated through molecular mechanics. The molecule has a complex globular shape with small internal cavities. This shape resulted mostly from the existence of polymethylene bridges connecting the aromatic regions. Molecular aggregates formed with the asphaltene and with nine resins from the same oil, and with n-octane and toluene, were also studied. The resins showed higher affinities for the asphaltene than toluene and n-octane and also exhibited a noticeable selectivity for some of the external sites of the asphaltene. This selectivity based on the molecular recognition of the site depends on the fit between the resins and the site of the asphaltene. The selectivity explains why resins of one oil may not solubilize asphaltenes from other crudes. An analysis of the changes in the enthalpic and entropic contributions to the free energy showed that both contributions should be considered when the stability of the asphaltene and resin molecular aggregates is examined.