AbstractThe thermal decomposition of polyethylene terephthalale has been examined by healing isothermally in a high‐mass reaction chamber and collecting the evolved acetaldehyde on a GC column for quantization. Kinetics were determined at lime intervals up to 1 h for temperatures 225°C, 240°C, 250°C, 260°C, 265°C, 270°C, 280°C, and 300°C. The rate of acetaldehyde evolution was found to be directly proportional to time of reaction at each temperature, thus fitting a zero‐order reaction. The Arrhenius plot of the data yielded a straight line above the melting point with a slope different from the straight line for reaction rates below the melting point. The change in slopes occurred around the crystalline melting point. The following thermodynamic values were found above and below the melting point, respectively:EA, 37.5 and 8.2 kcal/mol; ΔSA, + 1.5 and −54 e.u.;A, 0.68 × 1014and 0.43 × 102sec−1. Possible mechanistic implicat