Polyesterimide (PEI) resins are used for the preparation of PEI varnishes, for copper wire enameling. They are usually manufactured from dimethyl terephthalate (DMT), ethylene glycol, bisphenol A (BPA), tris(2-hydroxyethyl) isocyanurate, trimellitic anhydride, and 4,4'-diaminodiphenylmethane. The synthesis involves condensation and imidization. DMT and BPA were partially or completely replaced with the polycyclic components: dimethyl 2,6-naphthalenedicarboxylate (NDC) and (R,S)-1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane (TMC), respeclively, to enhance the heat resistance and the thermal stability. The effect of NDC and TMC on the solubility, the glass transition temperature, the course of the thermal decomposition, and the properties of magnet wires, enameled with the thus-modified PEI varnishes, were investigated. (c) 2006 Wiley Periodicals, Inc.