The polymerization of α-methylstyrene has been investigated using the “H2O”/SnCl4initiating system in the presence of 2,6-di-tert-butylpyridine (DtBP) in CH2Cl2solvent at −60°C. In the absence of DtBP and at low DtBP concentrations conversions are about 100%, but with increasing amounts of DtBP conversions drop to 3–5%. Polydispersity decreases with increasing [DtBP], The [Mbar]nversus [DtBP] plot shows a maximum; thus [Mbar]nincreases from ∼70,000 to ∼120,000 by increasing [DtBP] from 6 × 10−7to 4.8 × 10−4M and decreases to ∼ 10,000 on further increasing DtBP. The effect of DtBP on this polymerization system can be explained by assuming nonprotic initiation, termination only by DtBP, depressed chain transfer to monomer, and complex formation between the propagating carbocation and DtBP.