The thermal degradation of polyisobutylene is characterized by kinetics consisting of four types of intramolecular hydrogen abstraction (back-biting) of primary (p) and tertiary (t) terminal macroradicals (Ri and R;) and the successive scission at the inner position of the main chain. This reaction affords four types of terminal monoolefins in the volatile oligomers. Assuming the reaction occurs competitively under a steady state regarding the on-chain macroradicals, the composition of the monoolefins are represented as the rate ratios of the respective back-biting processes. The rate ratio between the abstractions of different types of hydrogens (CH, and CH,) from the same type of macroradicals is expressed only by the rate constant ratio. The (TTD),/(TVD), and (TTD),/(TVD), ratios remain constant during degradation, independently of the decreases in volume and molecular weight of the reacting polymers and this tendency agrees fairly well with the kinetic expectation. This result suggests that back-biting depends only on the local motion of the reacting chain ends. The ratios between the abstractions of the same type of hydrogen from different macroradicals are expressed by the product of the rate constant ratio and the integrated macroradical concentration ratio ( [ R i ] / [ R ; ] ) . The observed values of (TTD),/(TTD), and (TVD),/(TVD), decrease with reaction time. This decrease results from the decrease in macroradical concentration.