Axially oriented poly(ethylene terephthalate) fibres and films with widely differing processing histories were prepared by varying the spinning speed in the case of fibres and the speed, temperature, medium and extent of drawing for both fibres and films. The samples were held at constant length and heated to temperatures between 60 and 220-degrees-C with simultaneous recording of their thermomechanical response for up to 20 s. Samples produced at low speeds developed low shrinkage stress, which decayed with time. On the other hand, samples produced at relatively higher speeds developed higher shrinkage stress, which showed little relaxation with time. The structural basis of these results is discussed, and the roles played by processing parameters highlighted.