The effect ofn-alkenes, i.e., propylene, 1-butene, 1-pentene, 1-hexane, and 2-octene, on the yield and molecular weight of polyisobutene obtained with AlCl3catalyst inn-pentane solvent at −78°C has been investigated. It has been found that the lower 1-alkenes are pure poisons; i.e., they decrease the yield (rate poisons) but do not affect product molecular weight. The rate-decreasing effect ofn-alkenes has been treated quantitatively by poison coefficients and their numerical values correlated with relative carbonium ion stabilities using the allylic termination concept. The mild molecular weight decreasing effect of 2-octene has been attributed to moderate transfer activity. The effect of temperature on the poison coefficients of 2-octene has been investigated and some insight into the activation energies of various elementary steps has been gained.