A novel terephthalic acid derivative based on benzocyclobutene, XTA, has been used as a comonomer in the preparation of thermally crosslinkable poly(ethylene terephthalate) copolymers. We have examined the rheological properties of a series of such polymers containing varying monomer percentages of the XTA comonomer : 0, 1, 5, 10, and 20. Incorporation of XTA into PET is found to cause dramatic changes in the temperature and time dependence of the rheological material functions, all indicating that the rate of crosslinking between chains in the melt increases as the XTA percentage is increased. The impact of thermal crosslinking on mechanical properties is also examined using dynamic mechanical analysis (DMA) on samples molded at a selected crosslinking temperature for times yielding varying levels of crosslinking extent. DMA results show that the activation energy of the beta mechanical relaxation increases significantly, indicating the importance of torsional mobility of the aromatic ring of PET.