The results of study of composition and physicochemical properties of ethylene production byproduct, namely, heavy pyrolysis tar (HPT) are reported. It is shown that HPT is a complex multicomponent mixture essentially of aromatic compounds and partially of unsaturated hydrocarbons. The composition and physicochemical properties of HPT in an industrial pyrolysis plant vary with time in a wide range even when the same type of feedstock is pyrolyzed. In view of this, for using HPT as a feedstock for producing carbon material of a fixed quality, it is necessary to stabilize its composition, structure, and properties over time. The potential paths of solving this problem, among which are production of HPT as one of the target pyrolysis products and preparation of HPT as an olefin production by-product for processing into carbon materials of a fixed quality, are discussed.