The problem concerning the determination of asphaltene molecular weight has been critically investigated. A size exclusion chromatography (SEC) analytical method has been developed using innovative calibration standards. The obtained results have been compared with those derived from vapor pressure osmometry (VPO). All the experimental data indicate that the number-average molecular weights (Mn) determined either by SEC or VPO are always different. Nevertheless such discrepancies are less pronounced using the new calibration curve instead of either the conventional polystyrene standards or other polymers such as poly(ethylene glycol). Furthermore it has been observed that VPO measurements, considered to give the most reliable Mn values when performed in a highly dissociating solvent, at high temperature and low concentration, fails when strong interactions among single molecules are present. SEC and VPO have also been employed to study association phenomena undergone by asphaltenes in solution. A correlation between molecular structure and degree of aggregation has been recognized indicating that aromaticity is primarily responsible for the importance of the observed intermolecular associations.