This work concerns the changes in structural order, which occur when amorphous polyethylene terephthalate (PET) is crystallised by drawing and then subsequent annealing. Real time wide angle X-ray fibre diffraction is used to obtain information about the microstructural changes taking place during drawing and subsequent annealing. The diffraction patterns obtained proved the existence of a liquid crystalline transient mesophase prior to crystallisation. The development of both the mesophase and the crystalline structure are also studied using small angle X-ray scattering during annealing of uniaxially drawn samples held at constant strain. These experiments proved the absence of any microstructure associated with the mesophase and that significant microstructural changes take place only when crystallisation starts to occur. (c) 2007 Elsevier Ltd. All rights reserved.