AbstractA dynamic TGA method was used to study the effect of the molecular weight of well‐defined polystyrene samples as well as the effect of heating rate and sample weight on the kinetic parameters of the thermal decomposition. It is shown that the maximum rate, the average specific rate, and the activation energy of the decomposition increase up to molecular weights of about 360,000 and then level off. The activation energy for the first part of the decomposition was 33 ± 5 kcal/mole and 50 ± 5 kcal/mole for the second part. An activation energy of 42 ± 5 kcal/mole was determined for the overall decomposition of polysty