Polymerization of 2-methyl-1,3-butadiene (isoprene), initiated by titanium chloride in combination with trifluoro- and trichloroacetic acid and their t-butyl esters as coinitiators, was studied in benzene, 1,2-dichlorobenzene and heptane at 20 °C. Curves of the dependence polymerization rate vs coinitiator concentrations exhibit in the first two media a minimum and a maximum, those in heptane exhibit an inflex only. This is due to the fact that the acids and the esters are partly consumed to carboxylate TiCl4 and to form mono- and dicarboxylates of titanium chloride. Other products are either HCl or the corresponding alkylchlorides whose co-catalytic activity is very low. It follows from kinetic data and from electric conductivity and infrared spectra measurements that active centers in the polymerization are complexes of oligoesters of halogenoacetic acids with TiCl4 and its carboxylates, in which the ester bond is either considerably polarized or dissociated. Free ions are formed during the polymerization, too, contributing to changes of electric conductivity; however, they do not influence the polymerization rate directly.