Kinetic studies on the decomposition of poly-p-isopropyl-α-methylstyrene samples with molecular weights ranging from 1.2 × 104to 6.9 × 105were carried out with the differential thermogravimetric technique. Changes in molecular weight distributions, at different temperatures, were studied by gel-permeation chromatography. The results show that the mechanism responsible for the decomposition is that of depolymerization. The order of the reaction was found to be one. The activation energy for the decomposition of polymer samples of series S (which are less branched in structure) decreases with increasing molecular weight. For the more branched samples of series F, the activation energy remains essentially constant with increasing molecular weight. For identical molecular weights, samples of series F require a a lower activation energy than do samples of series S. The proportions of the three steric forms, viz., isotactic, hetero-tactic, and syndiotactic, in the polymer, both before and after thermal treatments, did not change, suggesting that stereoregularity has no apparent effect on the decomposition of poly-p-isopropyl-α-methylstyrene.