The kinetics and products of the thermal degradation of polyethylene terephthalate (Terylene polyester) have been examined over a range of temperatures above the melting point of the polymer. The effects of oxygen, water and the initial molecular weight of the polymer have been investigated.The results on dried polymer are consistent with a random chain-scission process, which is accelerated by oxygen, and which produces one carboxyl group per split. It has an overall activation energy of 32 kcal/mole. Any water present in the polymer is used up virtually instantaneously at these temperatures in hydrolysis.