The mechanism of cationic olefin polymerization has been investigated by studying the effect of 2,6-di-tertbutylpyridine (DtBP) on the polymerization of isobutylene by the C6H5C(CH3)2Cl/BCl3initiating system. In the absence of DtBP, initiation is by cumylation, chain transfer to monomer is absent, and termination is by chlorination. In the presence of DtBP, conversions and molecular weights decrease as a function of DtBP concentration, indicating that while DtBP does not interfere with nonprotic initiation by the C6H5C⊕(CH3)2carbocation, it functions as a terminating agent. According to Mayo plots, chain transfer to monomer is absent with or without DtBP in the system; however, the ratio kt/kp(the slope of the plot) obtained in the presence of DtBP is ∼25 times larger than that obtained in the absence of this hindered base which also indicates termination by DtBP. The exclusive presence of CH2˭C(CH3) ∼ endgroups in polyisobutylene prepared in the presence of DtBP suggests that this base is able to abstract a proton from the propagating ∼C⊕(CH3)2center. In contrast, according to initiator efficiency data, DtBP does not abstract a proton from the initiating C6H5C⊕(CH3)2cation.