The colloidal properties of asphaltenes affects their solubility, reactivity and transport properties. We have been investigating the molecular basis for the aggregation of asphaltenes in toluene through the use of Small Angle Neutron Scattering (SANS), in which the hydrogen nucleus provides the strongest scatterer, and the contrast may be varied by deuterium replacement. The measured intensity curves can be fitted with a elongated particle. Both radius of gyration and molecular weight are significantly decreased upon increasing. temperature, indicating a disaggregation rather than a conformational change mechanism. Thus, the “true” molecular weight of asphaltenes is much less that that measured at room temperature, and appears to be ≤6000. The internal structure is probably complex enough so that a simple description in terms of colloidal or micellar structures is not granted.