The synthesis and the thermal stability of polyesteramides based on PET and nylon 2,T (PETA) using DMT, T2T-dimethyl (N,N＇-bis(p-carbo-methoxybenzoyl)ethanediamine and 1,2-ethanediol as starting materials has been studied. The catalysts that were used are tetraisopropyl orthotitanate, manganese acetate, zinc acetate, antimony trioxide and caesium carbonate. After a short melt polymerisation at 370-290 degrees C for 15 min, the molecular weight was increased under milder conditions by solid state postcondensation (220-250 degrees C). The thermal properties, inherent viscosity, melt stability at 320 degrees C in nitrogen of the resulting polymers (PET with 25 mol% amide) was studied. Using tetraisopropyl orthotitanate as a catalyst, a polymer is obtained with a reasonable molecular weight, but the polymer has a high degradation rate constant. Inactivation of the titanium catalyst by triphenyl phosphite decreased the degradation rate, but compared to the other polymers it is still high. By using managese acetate and zinc acetate in combination with antimony trioxide, a polymer with a reasonably high molecular weight and with a low degradation rate constant is obtained. The degradation is mainly caused by manganese acetate, the transesterification catalyst. Increasing the diamide concentration in PET results in a lower molecular weight, this effect being noticeable at concentrations higher than 10 mol%. The thermal degradation rate of PETA increased with increasing diamide content. (C) 2000 Elsevier Science Ltd. All rights reserved.