The effects of various allyl chlorides and alkyl halides on the overall yield of polymerization and molecular weight of polyisobutene have been investigated and expressed quantitatively by poison and transfer coefficients. The poison and transfer coefficients of halides have been compared with those obtained previously for corresponding hydorcarbons. The poison coefficients of halides and hydrocarbons can be treated formally in a similar manner (1/Wpvs. [X] plots linear for both classes); however, the appropriate transfer coefficients indicate fundamentally dissimilar transfer mechanisms in these systems (1/MWpvs. [X] plots linear for hydrocarbons, whereas 1/MWpvs. [X]1/2plots linear for halides). These results are discussed in terms of the allylic termination mechanism.