Three N,N’-bis(glycidyl ester imide)s of pyromellitic acid (diepoxides) were prepared and were used as chain extenders for poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PET). The typical reaction conditions for the coupling of the polyester macromolecules were heating with the chain extender under argon atmosphere above the melting temperature (280 degrees C for PET and 250 degrees C for PET) for several minutes. The characterization of the samples, obtained at variable residence times in the reactor, was based on solution viscosity measurements and carboxyl and hydroxyl end-group determinations. Two of the diepoxides used gave satisfactory results. Starting from a PET having intrinsic viscosity [eta] = 0.60 dL/g, and carboxyl content CC = 42 eq/10(6) g, one could obtain PET with [eta] = 1.15 dL/g and CC = 16 eq/10(6) g within 30 min at 280 degrees C. Analogous results were observed for PET. The hydroxyl content of polyester in all cases was increased. When the quantity of the chain extender used was higher than that theoretically required for its reaction with all carboxyl end groups of the polyester, this resulted in some gel formation indicative of crosslinking.