An effective inhibitor of the thermopolymerization of liquid pyrolytic products may be produced by a new method. The product, obtained by mild heat treatment of phenol byproducts from coke production in a mixture with turpentine and solvent, exhibits strong inhibiting properties in the heat treatment of pyro condensates and significantly exceeds the effectiveness of the initial phenols and the well known inhibitor ionol, which is a sterically hindered phenol.