Abstract <p>The microstructure and functional compositions of the asphaltenes of bituminous oils from the Ashalchinskoye (Permian), Usinskoye (Permo-Carboniferous), and Nurlatskoye (Devonian) oil fields and their high- and low-molecular-weight components were studied using infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. It was shown that asphaltenes from the Ashalchinskoye oil were characterized by a smooth surface, while asphaltenes from the Usinskoye and Nurlatskoye oils had rough and porous surfaces. The sizes of asphaltene nanoaggregates from the Usinskoye and Nurlatskoye oils were smaller than those of the Ashalchinskoye oil. At the same time, asphaltene nanoaggregates from the Ashalchinskoye and Nurlatskoye fields formed a disordered tangled structure. A distinctive feature of asphaltenes from the Usinskoye oil was the presence of more ordered layers characteristic of crystal-like formations. Asphaltenes of Ashalchinskoye and Nurlatskoye oils were characterized by increased degrees of aromaticity and branching of aliphatic substituents in their macromolecules and a high relative concentration of fragments with a sulfoxide group. Their high-molecular-weight asphaltenes were less aromatic than low-molecular-weight ones, and their composition had a lower tentative concentration of carbonyl and sulfoxide groups. In the structure of asphaltenes form the Usinskoye oil, the fraction of aliphatic fragments and fragments containing a carbonyl group was increased. High-molecular-weight asphaltenes of this oil also contained fewer sulfoxide and carbonyl groups but more aromatic fragments than their low-molecular-weight asphaltenes.</p>