Study of the ionization of cumyl chloride by boron trichloride was carried out using stopped-flow UV-visible and H-1 NMR spectroscopy in methylene chloride. Cumyl chloride is very weakly ionized to form the cumyl cation (lambda(max) = 330 nm) which is stable at low temperature (<-60 degrees C) with tetrachloroborate anion since no elimination could be observed. At that temperature, owing to the presence of traces of alpha-methylstyrene in the cumyl chloride solutions, the ionization step was immediately followed by the addition of this monomer, resulting in the formation of the dimeric (or oligomeric) cation (lambda(max) = 348 nm), which slowly cyclized into the corresponding indanic derivatives. At -65 degrees C, the equilibrium constant of ionization of cumyl chloride is much smaller than that of the chlorinated dimeric (or oligomeric) species. At room temperature, dehydrochlorination of the cumyl cation leading to alpha-methylstyrene was evidenced and the indanic dimer was the only final product.