Accurate molecular weight distributions are essential to the determination of the molecular structures of asphalt. Due to molecular complexity and interaction among the molecules of asphalt, precise measurement for the molecular weights of asphalt can not be achieved. In order to minimize the interactions of asphaltic materials, a petroleum asphalt sample was separated into saturates, naphthalene aromatics, polar aromatics, and asphaltenes. These Corbett fractions were then analyzed by gel permeation chromatography against nondispersive polystyrene standards. The molecular weights and the molecular size profiles were obtained. Among these Corbett fractions, naphthalene aromatics have the lowest number average molecular weight; saturates have the lowest weight average molecular weight and dispersity; and asphaltenes have the highest number and weight average molecular weights and dispersity.