This article discusses the characterization of arborescent (hyperbranched) polyisobutylenes (arb-PIBs) by size exclusion chromatography and differential scanning calorimetry, in comparison with linear PIB standards. The radius of gyration (< r(g)(2)>(1/2) = R-z), measured from the angle dependence of light scattering of high-molecular-weight arb-PIBs, was significantly larger than the hydrodynamic radius (R-h) from size exclusion chromatography/viscometry, and the R-h values were significantly smaller than R-h of linear PIBs. The glass-transition temperature of arb-PIBs having a branch molecular weight higher than the critical entanglement molecular weight was dependent on both the total number-average molecular weight and BR up to BR similar to 15. A modified Fox-Flory equation is proposed to describe the effect of architecture on the thermal transition. (c) 2006 Wiley Periodicals, Inc.