Reported are a pair of studies exploring the possible role of transesterification during the deformation of poly(ethy1ene terephthalate) (PET) near its glass transition temperature. SANS experiments on PET-D/PET-H blends were carried out on films drawn according to the method of Petermann and Gohil at 70 and 90Β°C. In order to avoid effects of orientation on the SANS pattern, the measurements were made on liquid solutions made from the drawn material. The results show a decrease in the D-sequence length upon deformation, while the molecular weight of the chain is unchanged. This result indicates that bond exchange has taken place. Based upon the SANS results, a n approximate activation energy in the 8-24 kcal/mol range is reported for bond interchange. T o test the hypothesis that transesterification may play a role in the mechanical behavior of PET near T,, creep measurements were carried out over a temperature range on either side of T,. Activation energies obtained from time-temperature superposition shift factors showed a dual behavior: above T, the activation energy matches those for diffusive processes in PET; below T, the activation energy coincides with activation energies reported for exchange reactions in this material. The role of annealing prior to deformation is also reported, and it is shown that the activation energy increases rapidly with increasing annealing temperature. 0