The cyclization of cis-1,4 polybutadiene in various solvents (mesitylene, xylene, toluene, benzene and cyclohexane) with the catalyst composed of CH 2=CHCH 2Cl-AlEt 2Cl was studied. The infrared spectra of the cyclized products were investigated. It was shown that the products produced in cyclohexane and mesitylene have infrared spectra identical with those of the original cis-1, 4 polybutadiene ... [Show full abstract] and the products obtained in other aromatics have infrared spectra different from each other and distinguishing with those of the parent cis-1,4 polybutadiene. The analyses of infrared spectra came to the conclusion that the molecules of aromatic solvent participate in cyclization of cis-1,4 polybutadiene at the given condition. A possible reaction scheme involving an electrophilic substitution of carbonium ions for Ar - H of aromatic solvents was proposed. Some experimental facts were explained with great satisfaction on the basis of the above mechanism.