Cis, cis-1, 3-cycloöctadiene was polymerized by titanium tetra-chloride-trichloroacetic acid in methylene chloride or toluene at -78°C. The polymerization was very fast at first but slowed down toward the end. When the polymerization was conducted in methylene chloride, it stopped completely after the fast polymerization ceased, and a portion of monomers was left unreacted. On the other hand, when toluene was used as a solvent, a slow polymerization was observed after the fast polymerization finished, and the polymerization went to completion. The conversion reached during the fast polymerization was lower as the initial monomer concentration was higher. These experimental findings were interpreted in terms of a strong interaction between the excess monomers and the catalyst to form an inactive monomer-metal halide adduct, which has been commonly observed with other cyclic dienes studied so far, such as cyclopentadiene and 1, 3-cyclohexadiene. To explain the effect of solvent, a reactivation of the initially formed monomer-metal halide adduct was invoked in the case of toluene as solvent, which has not been observed with other cyclic dienes.