The effect of various cyclic and bicyclic olefins on the yield (rate) and molecular weight of polyisobutene has been investigated. Cyclic olefins were found to be both rate poisons and transfer agents and the corresponding coefficients appear to be a function of ring size. Alkyl and aryl substitution on the ring increases both the poison and transfer coefficients. With bicyclic olefins, norbornene and norbornadiene both show moderate and similar poison coefficients, but only the former exhibits transfer activity. These coefficients have been discussed in terms of the allylic termination hypothesis and the known geometry and steric effects of the various rings studied.