An integrated study of the hydrocarbon composition of bituminous substances has been performed, and the amount of the natural radionuclides Th-232 and Ra-226 in the surface layer of Caspian Sea sediments has been determined. The Ra-226 concentration in the sediment increases with the increasing bitumen content. A comparative analysis of the amount of bitumen hydrocarbon structures in sediments and the Th-232 and Ra-226 natural radionuclides has shown that the Th-232 and Ra-226 concentration in the sediment increases with increasing amount of aromatic structures in chloroform-extracted bitumen. The concentration of Ra-226 in the sediments increases with the increasing amount of polycyclic aromatic structures and decreases with an increase in the amount of monocyclic substituted aromatics. This fact indicates the preferential accumulation of Ra-226 in sediments with a less reduced organic matter, which is typical of modern sediments in the early stages of diagenesis.