The polymerization of isobutylene (IB) in supercritical carbon dioxide (SC.CO2) has been investigated in the temperature range from 32.5 to 47.5-degrees-C at 139+/-3 bar. The In M(n) of polyisobutylene (PIB) versus 1/T plot gave DELTAHdouble-ended dagger(Mn,PIB) = - 14.1+/-1.5 kcal/mol (-59.1+/-6.3 kJoule/mol) and DELTASdouble-ended dagger(Mn,PIB)=-31.1+/-6.2 cal/mol.deg (-130.2+/-26.0 Joule/mol.deg). These quantities are quite different from those obtained in conventional liquid systems. The ceiling temperature (T(ceil)) was determined by linear extrapolation of the In M(n) versus 1/T plot to the molecular weight of the monomer. According to this procedure the T(ceil) of IB polymerization in SC.CO2 in about one molar solution at 139+/-3 bar is 88+/-9-degrees-C.