The controlled polymerization of indene initiated with cumyl chloride and tin tetrachloride in methylene dichloride solution shows first-order kinetics with respect to SnCl4, which implies the existence of simple equilibria for ionization and dissociation, contrary to the case of TiCl4. Experiments in the presence of a hindered base (2,6-di-tert-butyl-4-methylpyridine) allowed to estimate the propagation rate constant for paired and unpaired ions in the range of 5 x 10(4)-10(5) L mol(-1) s(-1) at -40 degreesC. A fast initial polymerization, which takes place in the absence of base, and is suppressed by the base, is attributed to the larger dissociation constant for the Cum(+),SnCl5- ion pairs than for the polyindenyl ones, yielding a transitory excess of free ions. The hindered base suppresses the formation of free ions, may act as a proton trap in the case of zero order transfer, and may also be involved in bimolecular termination.