The polymerization of isoprene in aromatic solvents, catalyzed by an ethylaluminum dichloride-titanium tetrachloride system, proceeds via a complex of the monomer with the catalyst, chain growth occurring when a monomer cation-radical is formed through the transfer of one electron from the monomer to the catalyst. The deactivation of the active species results from the dissociation of the complex polymer cation-radical to free ions. The over-all reaction is a nonstationary process during which the concentration of free ions increases and the concentration of the active species decreases. A reaction mechanism is proposed which leads to kinetic expressions that quantitatively describe the experimental data.