The synthesis and characterisation of a new type of high melting and fast crystallising amide units that can be used for copolymerisation have been studied. These bisester tetra-amide or TxTxT-dimethyl segments (T is a terephthalic unit and x = (CH2)(n) (n = 2-8)) can be synthesised in a two-step reaction. These segments are based on two-and-a-half repeating units of nylon-x,T. The structure of the products was confirmed by NMR. The melting temperature and enthalpy of xTx-diamine and TxTxT-dimethyl and -diphenyl were determined by DSC. The melting temperature increases with decreasing length of x. For odd x the melting temperature is lower. The melting temperature and enthalpy of the products decrease with decreasing purity. (C) 2003 Elsevier Ltd. All rights reserved.