The overall activation energy of the thermal degradation of polyisobutylene has been measured using factor-jump thermogravimetry to be 206±1 kJ/mole over the range 365 to 405° in N2 at 800 mm Hg pressure and flowing at 4 mm/s over the sample. This is consistent with some values reported for thermal degradation in vacuum and in solution. In 5 mm Hg of N2, an apparent activation energy of 218±2 kJ/mole was found, and in vacuum the apparent activation energy is 238±13 kJ/mole. Troublesome bubbling made the vacuum values difficult to measure. Substitution of reasonable values for the activation energies of initiation,Ei, termination,Et, and the activation energy,Ea, for vacuum degradation in the equationEa=Ei/2Ed-Et/2 yields an activation energy Ed=84 kJ/mole for the unzipping reaction. This equation presupposes a degradation mechanism of random initiation, unzipping, and bimolecular termination. Substitution of reasonable values for the heat of polymerization, ΔH, in the definition ΔH=Ep−ed suggests that the activation energy of the polymerization reaction at 375° is approximately 30 kJ/mole.