Alternating copolyesteramides (APEA's) consisting of mixtures of 1,4-butyleneterephthalamide ester (N,N'-bis(p-carbomethoxybenzoyl) diaminobutane) and 1,4-butyleneisophthalamide ester (N,N' -bis(m-carbomethoxybenzoyl) diaminobutane) polymerized with ethanediol were synthesised in an ethanediol solution. The bisesterdiamides were prepared from 1,4-butanediamine and dimethyl terephthalate or dimethylisophthalate respectively. They were analyzed by H-1 n.m.r., differential scanning calorimetry (d.s.c.) and high pressure liquid chromatography (h.p.l.c.). The melting behaviour of the copolyesteramides was studied by d.s.c. The dynamic mechanical properties were investigated on injection moulded bars using dynamic mechanical thermal analysis (DMTA). The melting temperature decreased with increasing 1,4-butyleneisophthalamide ester content. The crystallisation rate, the modulus above the glass transition temperature and the glass transition temperature hardly changed. The APEA of 1,4-butyleneisophthalamide ester and ethanediol was amorphous. The lowering of the melting temperature of the copolymers was probably caused by a decrease in lamellar size. The high crystallisation rate and the high physical crosslink density of the copolymers must be ascribed to the presence of 1,4-butyleneterephthalamide ester units which are able to crystallise fast and to form strong crystallites. (C) 1997 Elsevier Science Ltd.