1. The kinetics of the polymerization of phenylacetylene on catalysts (α, δ-TiCl3)-(C3H5)3Al in heptane and benzene medium was studied by a dilatometric method. 2. The influence of the concentration of the components of the catalytic system on the rate of polymerization of phenylacetylene was investigated, and the observed constants, related to the concentration of the catalyst k′ (liters/g·sec), were determined. 3. The temperature dependences of the polymerization of phenylacetylene (40–70°) were studied and the observed activation energies (E) were calculated. The values of E and k′ for styrne were obtained for comparison. 4. In the polymerization of phenylacetylene in heptane, the formation of a substantial amount of cyclic trimer, the yield of which depends on the concentration of (C2H5)3Al, is observed; chiefly a linear polymer is formed in the case of polymerization in benzene. 5. A mechanism of the formation of cyclic products was proposed.