The products of hydrothermal catalytic conversion of extra-heavy Ashal'chinskoe oil at temperatures of 210, 250, and 300A degrees C in a closed system with different amounts of water in the presence of the natural catalyst iron oxide (hematite) have been studied. It has been shown that the degradation of high-molecular-mass components of the extra-heavy oil, such as benzene- and alcohol-benzene-extractable resins and asphaltenes, leads to a generation of new light fractions that were absent in the initial crude oil. A difference between the oil components in stability to the conversion processes has been shown. The most significant changes are observed for the reduction in the amount of alcohol-benzene resins, which is accompanied by an increase in aromaticity and the extent of oxidation. In asphaltenes, the concentration of free radicals (R*) increases and the concentration of tetravalent vanadium (V+4) decreases, changes that are accompanied by enhancement of structure carbonization and a reduction in their molecular mass. It has been shown that coking includes the step of formation of asphaltenes followed by the conversion of a part of them into high-carbon-content toluene-insoluble substance of the carbene and carboid type, which precipitate together with coke from the petroleum disperse system when a certain concentration is reached. Changes in the structure of the hematite catalyst has been also revealed.