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The main laws governing the thermal transformations in vacuum of phenylquinoxalines, which are compounds which simulate the elementary units of polyphenylquinoxalines are studied. These reactions involve a radical-chain mechanism. The initiation stage is determined by the mobility of the hydrogen atoms of the aromatic ring conjugated with the heterocyclic ring. Chain propagation involves radical polymerization at the CN bonds, with the formation of oligomers containing linear and ladder structures consisting of quinoline and benzimidazole fragments. The main volatile product of the thermal degradation of the heterocyclic ring is toluene, formed as the result of transformation of the phenylquinoxaline rings into benzimidazole rings. The pyrolysis is completed on accumulation of insoluble N-containing carbonized residue (60 wt%) and evolution of low molecular degradation products, i.e. 3-phenylbenzimidazole, toluene, benzonitrile, benzene, and hydrogen.