The thermal stability of a poly(ester amide) copolymer (PBTA) based on poly(butylene terephthalate) (PBT) and nylon-4,T with the diamide of butanediamine and dimethyl terephthalate (N,N'-bis(p-carbomethoxybenzoyl)butanediamine) and homopolymer PBT was studied. The development of inherent viscosity and endgroup concentration was determined during prolonged condensation reactions at 255-275°C. Analysis of the kinetics lead to degradation rate constants for PBT and PBTA with 20 mol% amide (PBTA20). The degradation rate of PBTA20 was comparable to that of PBT, thus fl-elimination of the ester groups is the main degradation mechanism in PBTA. At high temperatures ester-amide interchange reactions also take place in PBTA. The change in the melting temperature of PBTA has been related to the decreasing uniformity of the amide segment length. The decomposition was further studied by thermogravimetry and mass spectrometry.