The influence of 2,6-di-tert-butylpyridine (DtBP) on the polymerization of α-methylstyrene (αMeSt) induced by the “H2O”/BC13initiating system in the -20 to −60°C range has been studied in detail. Adventitious H2O (“H2O”) is the initiating cationogen and initiation most likely proceeds by a concerted route in the absence of free protons or the acid H⊕BC13OH⊕Polymerizations are extremely rapid and kinetic termination is absent (conversions are 100%) in the absence of DtBP. In the presence of DtBP, polymerizations are still very fast; however, conversions are reduced. Significantly, conversions increase with decreasing temperatures, which suggests the operational presence of terminative proton entrapment. Molecular weights increase with decreasing temperatures in the presence and absence of DtBP and the slopes of the linear Arrhenius plots (In [Mbar]wversus 1/T) are parallel; the molecular weights obtained in the presence of DtBP are close to a factor of 10 higher than those produced in the absence of this hindered pyridine. The virtual identity of the slopes of Arrhenius plots indicates close similarity between the nature and rate of molecular-weight-determining events in the absence and presence of DtBP, i.e., kp/ktr,mand kp/ktr,G⊕DtBP profoundly affects molecular weight dispersity: [Mbar]w/[Mbar]n= 3.0–4.0 in the absence of DtBP whereas [Mbar]w/[Mbar]n= 1.5 -1.8 in the presence of DtBP. The number w of polymer molecules formed (yield/[Mbar]n) in the absence of DtBP whereas [Mbar]w/[Mbar]n= 1.5–1.8 in presence of DtBP. The number of polymer molecules formed (yield/[Mbar]n) = 1.5–1.8 in the presence of DtBp. The number of polymer molecules formed (yield/[Mbar]n) in the absence of DtBP decreases with decreasing temperature while those formed in the presence of DtBP remain constant. According to Mayo, (1/[Mbar]nversus 1/ [M] plots chain transfer to monomer in the peresence of DtBP is vey low (ktr,m/kp= 6.4 × 10−4and 2.8 × 10−4at −30 and −50°C) but not zero. Conceivably two kinds of chain transfer to monomer reactions may exist (direct and indirect) and only one (i.e., the indirect one) may be trappable by DtBP. The effect of [DtBP] on the percent converstion and [Mbar]nwas investigated: Above a fairly well defined [DtBP], neither conversions nor [Mbar]n's were affected by [DtBP]. With increasing [DtBP] molecular weight dispersions rapidly decrease and [Mbar]w[Mbar]n's seem to level off at ∼ 1.5 at relatively high [DtBP]. Changing the polarity of the solvent characteristically affects the mechanism of α MeSt polmerzation in the presence and absence of DtBP results in a strong increase in [Mbar]w/[Mbar]n(from ∼ 1.5 to ∼ 4.0), in the presence of DtBP [Mbar]w/[Mbar]nremains virtually unchanged at ∼1.5.