Kinetic studies on the decomposition of poly-α-methylstyrene samples with molecular weights ranging from 3.7 × 103to to 2.0 × 105have been carried out with the differential thermo-gravimetric technique. Changes in molecular weight distributions with decomposition, at different temperatures, have been studied by gel-permeation chromatography. A depolymeriza-tion mechanism was shown to be responsible for the decomposition phenomenon. The order of reaction for this depolymerization reaction was found to be one. The values of the activation energy for samples of different molecular weights showed no specific trends; however, it would appear that polymers with relatively higher molecular weights have lower activation energies of decomposition. The proportions of the three steric forms, viz., isotactic, heterotactic, and syndiotactic, in the polymer before and after thermal treatments did not change, suggesting that stereoregularity has no apparent effect on the decomposition of poly-α-methylstyrene.