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The composition and structure of nitrogen-containing bases isolated from the maltene fraction of the liquid products of conversion of asphaltenes of bituminous oil from the Ashalchinskoye oilfield in a supercritical hexane stream at 450°C and 15 MPa are investigated. It is shown that they are represented by a mixture of high molecular weight and low molecular weight compounds, where high molecular weight components prevail. Compared to the initial asphaltenes, their mean molecules are characterized by smaller sizes associated with a decrease in the number of structural blocks (ma = 2.3 versus 3.4), in the composition of which the number of saturated cycles is significantly reduced (Rn* = 2.8 versus 6.0). The size of the aromatic core remains virtually unchanged. (Ra* = 2.9 versus 3.4). At the same time, the internal arrangement of aromatic cores in the naphthenoaromatic system of the structural block (Cα* = 4.5 and 5.6) is preserved, which causes a high degree of screening of the nitrogen atom and, as a consequence, thermal stability of the compounds. Using the GC–MS method, C2−C6 quinolines, C2−C4 benzoquinolines, C1 dibenzoquinolines, and C16H33NO, C18H35NO, C18H37NO aliphatic amides have been identified in the nitrogen-containing bases of maltenes. [ABSTRACT FROM AUTHOR]