The role of dimethyl ditelluride (MeTe)(2) for the organotellurium-mediated living radical polymerizations (TERPs) of styrene (St) and methyl methacrylate (MMA) was kinetically studied. For both St and MMA, there was a rapid reversible activation-deactivation process mediated by (MeTe)(2), i.e., P-TeMe + MeTe center dot reversible arrow P-center dot + (MeTe)(2): (MeTe)(2) worked as an efficient deactivator of the propagating radical P-center dot, and the radical MeTe center dot worked as a highly reactive activator of the dormant species P-TeMe. This rapid reversible process accounted for the dramatic improvement of the polydispersity controllability with the addition of even a small amount of (MeTe)(2) for these polymerizations.