Athabasca asphaltene has been separated according to molecular weight on Bio-Beads SX-1 gel. The number-average molecular weights of the five arbitrary fractions obtained by this fractionation range from 1200to 17000. Thechemical, spectral and thermal propertiesof thefractionsareall similar but their polymer-forming propensities are markedly different. The significance of this latter property, which is defined in terms of the amount of CH,CI,-insoluble material produced upon thermolysis at 3Oo"C, increases rapidly with increasing molecular weight of the fraction. In contrast, the whole asphaltene does not form polymer at 3OOC under the same conditions and it is concluded that the chain propagating steps are terminated by a variety of inhibitors that are contained in the asphaltene agglomerate. During gel permeation chromatography separation the clay present in the asphaltene concentrates in the higher-molecular-weight fractions. This affinity to attract the clay is thought to be related to the physical, and not the chemical, properties of these higher-molecular-weight materials. The clay also exerts a catalytic effect on the polymerization of the asphaltene fractions which is most pronounced in the highest-molecular-weight fraction and gradually decreases with decreasing molecular weight.