Polyesteramide copolymers (PBTA) based on polybutylene terephthalate (PBT) and the diamide of butane-diamine and dimethyl terephthalate (N,N¿-bis(p-carbo-methoxybenzoyl)butanediamine) were studied. The amide content in PBTA was varied from 0¿50 mol%. The melting and crystallization behaviour was analysed with differential scanning calorimetry. The degree of undercooling is taken as a measure for the rate of crystallization. The torsion modulus as a function of temperature and the position of the glass transition temperature (Tg) were studied with dynamic mechanical analysis. On injection moulded samples the tensile properties at RT were determined. The crystalline spacings were studied with wide angle X-ray diffraction. In the PBTA only one Tg could be observed which means that the amorphous phase was homogeneous. The crystalline spacings of PBT and Nylon 4, T were not the same and the PBTA has something of both, which means that the ester and amide repeat units are not isomorphous. Despite the absence of isomorphous crystallization the melting temperature increased nearly linearly with the amide content, the crystallinity remained high, and very surprisingly the rate of crystallization even increased. Also in polyethylene terephthalate these diamide segments increased the rate of crystallization. A model is proposed with the diamides as homogeneous nucleation sites (adjacent crystallization) to explain the fast crystallization behaviour of these copolymers.