The products of thermal degradation of polyisobutylene are successfully simulated according to a radical chain reaction model including diffusion-controlled termination reactions of primary and tertiary terminal macroradicals (R(p) ., and R(t) .) and volatile small radical (S .). The model proposed consists of the following three steps : (1) end and random initiation reactions, (2) depropagation consisting of depolymerization and intramolecular and intermolecular hydrogen abstractions followed by beta scissions, and (3) diffusion-controlled termination consisting of bimolecular reactions between respective macroradicals and vaporization of volatile radicals. The molecular weight (M) dependencies of rates of the end initiation and termination are evaluated by M(-1) and M(-n), respectively. Assuming that the reaction occurs competitively under steady state conditions regarding the respective radicals, their concentrations could be approximately expressed as a function of M. The observed values of the compositions of the studied components of the volatile oligomers and functional groups of the nonvolatile oligomers formed by the degradation at 300 degrees C, are consistently traced by simulation using the above model when the value of n is about 2 for the self-diffusional motion of the reacting radical in the molten polymer matrix. Rates of decreases in concentration are of the order : S .>R(p) .much greater than R(t) .. This results from an increase in the rate of termination with a decreasing molecular weight of the matrix as the reaction proceeds.