Asphaltenes derived from oxidized asphalts are separated into five fractions that differ in solubility. The amount of the low-molecular-mass fraction in the asphaltenes does not exceed 20%, and that of the high-molecular-mass fraction is above 50 wt %. The high and low-molecular-mass fractions of the asphaltenes are studied by EPR spectroscopy, thermal analysis, and calorimetry. It is found that the high molecular-mass fraction of the asphaltenes is characterized by a high amount of condensed aromatic structures; it has a tendency to form mesophases with different thermal stability and is responsible for the formation of ordered structures in asphalt. It is revealed that an increase in the amount of the high molecular-mass fraction in asphaltenes of oxidized petroleum asphalts leads to a decrease in their stability, which has an adverse effect on the stability of physicochemical characteristics and quality of the commercial products. It should be noted that this dependence is more pronounced for structured asphalts.