It has been found that the amount of the asphaltene fraction soluble in a 1.85: 1 heptane-toluene solvent mixture is at most 12%. The major portion of asphaltenes are insoluble in this mixture. The molecular weight of soluble asphaltenes is lower than that of the insoluble ones. According to thermogravimetric analysis data, the low-molecular-weight asphaltenes are thermally less stable, because their aromaticity is lower than that of the high-molecular-weight asphaltenes, and the number of aliphatic and heteroatomic substituents in them is higher. The aromaticity and the number of paraffinic substituents in the low-molecular-weight asphaltenes are close to those of the resins according to the FT-IR spectroscopy data. Similar to the resins, the total amount of heteroatomic substituents in the low-molecular-weight asphaltenes is higher than that in the high-molecular-weight asphaltenes.