The thermal decomposition and the glass transition temperature (T) of polyisobutylene (PIB) were studied with a differential scanning calorimeter (DSC). The undecomposed and the decomposed polymers were analyzed by gel permeation chromatography for molecular weight distributions and by DSC for changes in the thermal properties, e.g., T. In the isothermal decomposition of PIB, weight loss, a, is independent of the molecular_weight (M) of the polymer. Random chain-scissions reduce [Mbar]wand [Mbar]nto average values of 10 × 103and 5.0 × 103respectively, and [Mbar]nto average value of 10 × 103and 5.0 × 103respectively, after isothemal treatments for 50 min intervals at temperatures ranging from 250 tp 375°C. The value of α is depolymerization controlled and the PIB is not quantitatively converted into monomer. Oligomers with a degree of polymerization of 12 are important products of the decomposition reactions. The activation energy of decomposition of PIB was found to be 184 kJ/mol, a value somewhat lower than that reported in the literature. Tgevalues (at q = 1 K/min) of undecomposed and decomposed PIB samples fall within ± 3.0° of 200 K in excellent agreement with reported values obtained by other means.