The effect of branched alkenes on the rate (yield) and molecular weight of polyisobutene has been investigated. Monosubstituted branched ethylenes, i.e., 3-methyl-1-butene, 3,3-dimethyl-1-butene, and 4-methyl-1-pentene, were found to be virtually inert and did not affect the polymerization of isobutene under our experimental conditions. 2,2-Disubstituted branched ethylenes, i.e., 2-methyl-1-pentene, 2-ethyl-1-hexene, 2,4,4-trimethyl-1-pentene, and 2,4,4-trimethyl-2-pentene, were rate poisons and strong transfer agents. The steric configuration of substituents in the vicinity of allylic hydrogens strongly influences the poison and transfer coefficients of these materials. These coefficients have been discussed quantitatively in terms of the allylic termination theory.