Tensile drawing experiments on poly(ethylene terephthalate) (PET) have shown that there is no significant strain-induced crystallisation until draw ratios around 2.3. The actual onset of strain-induced crystallisation depends on the deformation rate, and it occurs at lower draw ratios for higher draw rates. The development of strain-induced crystallisation has a significant impact on the relaxation behaviour of PET. Online birefringence measurements, during the relaxation of PET drawn to different draw ratios showed that: (a) at low draw ratios, the orientation relaxes over long periods of time and Fourier transform infrared (FTIR) spectroscopy showed that the overall orientation falls after relaxation; (b) at higher draw ratios, when significant strain-induced crystallisation has occurred, the orientation decreases over short times, of the order of 10 s and remains constant thereafter. This behaviour is probably due to the crystallites that lock in the extension of the chains in the amorphous material. Post relaxation FTIR measurements showed that the orientation increases due to annealing effect. Differential scanning calorimetry (DSC) showed that there is an increase in the crystalline fraction after relaxation for all draw ratios, which is probably due to the conversion of oriented amorphous material (trans conformers) into the crystalline phase.