Non-volatile oligomers obtained by the thermal degradation of polyisobutylene were finely fractionated and the distribution of the functional groups of each fraction was characterized by a kinetic approach including the intermolecular hydrogen abstraction of primary and tertiary terminal macroradicals (R-p and R-t) and volatile small radicals (S-.), followed by beta scission. The observed value of the composition ratio [TTD][NTTD], which is related to the ratio of the competitive beta scission rates of a secondary on-chain macroradical, increases monotonically with decreasing molecular weight (M) of the fractions. This result shows clearly that the reactivity for beta scission depends on the segmental motion of the reacting radicals. With respect to the composition ratio [t- Bu]/[iPr], which is related to the concentration ratio [R-p(.)]/[R-t(.)], the observed value is roughly constant with M for M > M-c, but decreases with decreasing M for M < M-c, where M, corresponds approximately to the characteristic molecular weight for the entanglement of molten polymer. This ratio for M > M-c decreases with decreasing average molecular weight of the molten polymer matrix, owing to an increase in the rate of diffusion-controlled termination. For M < M-c, the value of the exponent a determined from the slope of the double logarithmic plots of [t-Bu]/[iPr] against M is about 0.5 in all cases. This suggests that the hydrogen abstraction of S depends on the chain dimension of unentangled polymer. It is confirmed from these results that the degradation reaction of all the polymer molecules in an homogenous reaction medium is entirely affected by the segmental motion and self-diffusional motion of the degrading polymers, and the effect of the chain dimension is observed only for unentangled polymer molecules during the degradation.