The formation of functional groups of non-volatile oligomers by the thermal degradation of polyisobutylene is characterized by a kinetic approach including intermolecular hydrogen abstraction of primary (p) and tertiary (t) terminal macroradicals (R(p)(.) and R(t)(.)) and volatile small radicals (S-.), followed by beta-scission. By assuming in the kinetic analysis that the reaction occurs competitively under a steady state regarding the on-chain macroradicals, various composition ratios for the functional groups can be represented in terms of the rates of respective hydrogen abstraction. The ratio between the tert-butyl endgroup (t-Bu) and the isopropyl endgroup (i-Pr), which corresponds to that between the abstraction rates of R(p)(.) and R(t)(.), is expressed by the product of the rate constant ratio and the integrated macroradical concentration ratio ([R(p)(.)]/[R(t)(.)]) The observed value of the ratio [t-Bu]/[i-Pr] decreases with reaction time. This is induced by a decrease in the molecular weight (M) of the reaction medium. The molecular weight dependence (M(a)) of [t-Bu]/[i-Pr] is expressed by that of [R(p)(.)]/[R(t)(.)]. The value of exponent a was determined to be about 1.2 and 0.7 at 300 and 320 degrees C, respectively, from the analysis of data at various reaction times. These values are roughly consistent with average values (1.0 and 0.9) of the same power-law exponent for the volatile oligomers. These results support the hypothesis that the concentrations of the respective radicals decrease in different ways.