The effect of various conjugated dienes, e.g., butadiene, isoprene, 1,3-pentadiene, 2,3-dimethylbutadiene, 2,5-dimethyl-2,4-hexadiene, cyclopentadiene, 2-methylcyclopentadiene, and 1,3-hexadiene, on the rate of polymerization (yield) and molecular weight of polyisobutene has been investigated. The overriding effect of dienes is rate retardation (poisoning) and a lesser measure of molecular weight depression (transfer activity) (exception, 1,3-pentadiene). The effects have been expressed quantitatively with poison and transfer coefficients and their values have been discussed. The poison coefficients indicate that the order of stabilities of substituted allyl carbonium ions is