The chemical structure of the non-volatile oligomers (M(n) = 2600-9000) isolated from the polymer residues obtained by the thermal degradation of polyisobutylene at 300 and 320 degrees C was determined by 400 MHz H-1 n.m.r. spectroscopy with regard to the reactive end groups. The functional groups formed in the degradation process are as follows : tert-butyl end group (t-Bu), isopropyl end group (i-Pr), terminal trisubstituted double bond (TTD), terminal vinylidene double bond (TVD), and non-terminal trisubstituted double bond (NTTD). The average number of TTD and TVD per molecule (f(t)) is in a range from 1.46 to 1.64. This value suggests that 53-67 mol% of the non-volatile oligomers are telechelic oligomers having both TTD and/or TVD, and 39-29 mol% of the oligomers are macromonomer-like oligomers having a double bond at one chain end. Both t-Bu and i-Pr are produced by the intermolecular hydrogen abstraction of primary and tertiary terminal macroradicals, and the subsequent beta scission of the resulting on-chain macroradicals at the skeletal C-C bond of the main chain yields TTD and TVD. The formation of telechelic oligomer is interpreted by a major contribution of the hydrogen abstraction of volatile radicals, in addition to a minor contribution of the hydrogen abstraction of terminal macroradicals.