The first carbocationic polymerization of isobutylene (IB) in supercritical carbon dioxide (SC.CO2) has been accomplished. It was demonstrated that in CO2 at 32.5-degrees-C and approximately 120 bar the 2-chloro-2,4,4-trimethyl-pentane (TMPCl)/SnCl4 and TMPCl/TiCl4 initiating systems lead to approximately 30% IB conversion, and gave polyisobutylenes (PIB) with M(n) approximately 2000 and M(w)/M(n) approximately 2.0. This is the highest temperature IB was ever polymerized to reasonably high molecular weight products. Polymerizations at 32.5-degrees-C under similar but conventional (nonliving) conditions in the absence Of SC.CO2 WOUld yield only very low molecular weight oligomers (approximately tetramers). The structure of the PIBs obtained in SC.CO2 is virtually identical to those obtained at much lower temperatures in conventional liquid-phase systems indicating the presence of chain transfer to monomer in both systems. In contrast to TMPCl initiated polymerizations, the 1,3-bis-(2-hydroxy-2-propyl)-5-tert-butylbenzene (HPBB) initiator in conjunction with BCl3 and SnCl4 yields only oligomers (M(n) approximately 500) in SC.CO2.