Four current commercial antioxidants (Naugard 445, Ethanox 330, Irganox 1098 and a blend of Irganox 1098 and 1019) were added during synthesis of poly(ethylene terephthalate) in order to test their thermo-oxidative stability and eventually to obtain more thermostable products during synthesis and processing. Their stabilization effect was evaluated by DSC analysis in nitrogen and air, using as criteria the stabilization coefficient and the induction period of oxidation. At low concentrations (0·03% and 0·10%), under the conditions used, better results were obtained with Naugard 445 and Ethanox 330, while at higher concentrations (0·5% and 1·0%) Irganox 1098 and the blend of Irganox 1098 and 1019 were better than the others.