We have studied regularities of the epoxidation process of the С 9 fraction of liquid by-products of the pyrolysis of hydrocarbons and co-oligomers based on the С 9 fraction with peracetic acid. It is established that the use of the specified system makes it possible to achieve high values of conversion for double bonds and the selectivity of epoxidation process, sufficiently high yield of epoxy-containing co-oligomers. We established the possibility to use the С 9 fraction of liquid by-products of hydrocarbon pyrolysis (the accompanying product of ethylene production) as a raw material for obtaining epoxides, which is a rational method in order to dispose of the fraction. We analyzed experimental dependences of reagents consumption and the formation of epoxide in the reaction of the С 9 fraction epoxidation. It was established that the resulting values for selectivities of the epoxidation process by double bonds when obtaining epoxy-containing co-oligomers are the highest in the case of epoxidation of co-oligomers, synthesized by the heterogeneous catalytic co-oligomerization of the С 9 fraction S9 hydrocarbons using, as a catalyst, activated bentonite clay. The synthesized epoxy-containing co-oligomers retain residual unsaturation. It is obvious that the unsaturated bonds of styrene, vinyl toluene, α-methyl styrene, allylbenzene links at the ends of a co-oligomer macromolecules are easily epoxidized. The selectivity of epoxidation of co-oligomers, obtained by different methods of co-oligomerization of the С 9 fraction hydrocarbons (homogeneous catalytic, heterogeneous catalytic, initiated, and thermal) is different (60.2‒96.1 %) and depends on the nature of the starting co-oligomer. Using the methods of 1 H NMR-, Raman-, and infrared spectroscopy, we confirmed the high content of epoxy groups in the composition of epoxy-containing co-oligomers and the progress of the epoxidation reaction. Applying the method of 1 H NMR spectroscopy, we established the presence of epoxy groups formed based on the resulting vinyl double bonds of co-oligomers. Due to the content of oxirane oxygen and a high reaction ability of the oxirane ring, epoxy-containing co-oligomers can act as the raw material for subsequent obtaining of various chemical substances: glycols, carbonyl compounds, plasticizers, polymers.