Oil annealing produces changes in fibre structure of drawn PET not found in air annealing. The results are similar to those found in drawn nylon 66 but there are some differences especially in the effect of annealing on the small-angle intensity. It is suggested that the shrinkage of drawn PET in oil is due to chain folding, or fold regularization of crystals, and to relaxation in the non-crystalline regions. © 1969.