Relevance. Thermal destruction is one of the main processes for upgrading heavy oils provided in the technological workflow. It is difficult to process such oils using existing basic technologies due to their high resin content. Hence, obtaining and summarizing information on the effect of thermal action on the composition and structure of organic nitrogen bases in resin components of heavy oils is of great importance for the development of innovative technical solutions for their rational use. The need for this work is due to their negative impact on the efficiency of primary processes of heavy oil refining and further catalytic upgrading of distillate fractions. Aim. To obtain comparative characteristics of the structural-group and molecular composition of organonitrogen bases isolated from resins of heavy oils and liquid products of their thermal destruction. Methods. Autoclave thermolysis in nitrogen atmosphere, complexation, extraction, liquid-adsorption chromatography, elemental and functional analysis, molecular weight measurement, ¹H NMR spectroscopy, structural-group analysis, gas chromatography-mass-spectrometry. Results. The authors have studied the composition and structure of organic nitrogen bases isolated from resins of bituminous oils and from liquid products of conversion of resinous components. These oils differ in the age of host rocks, the content of resin components, nitrogen compounds, and the content of bases in their composition. It was found out that organic nitrogen bases of the original and thermally converted resins are high- and low-molecular compounds, which are mainly represented by high molecular weight bases precipitated by hydrogen chloride. As a result of thermal cracking, the relative content of hexane-soluble compounds increases, while the content of hexane-insoluble compounds decreases dramatically in the composition of these bases. The method of structural-group analysis showed that the main thermal transformations of high- and low-molecular bases of resins of bituminous oils are the destruction of alkyl substituents and cyclization of shortened alkyl chains with the formation of naphthene rings. Compositionally similar alkyl-substituted quinolines, benzoquinolines, azapyrenes, thiophenoquinolines and benzothiophenoquinolines were identified among the low-molecular organic nitrogen bases of the original resins and liquid products of their cracking. The content of benzoquinolines in the composition of the identified bases decreases while the relative content of quinolines increases more noticeably in the course of the thermolysis of resins, an increase in the content of compounds containing a smaller total number of carbon atoms in the alkyl substituents of the aromatic cycles is observed for organic nitrogen bases of the conversion products.