The mechanical properties of Poly ethylene terephthalate (PET) were studied over several decades of strain rate and a temperature range of 263 K-453 K. Tests were carried out in the range 10(-3)-10(4) s(-1) using a conventional Hounsfield machine and two high strain rate test systems. Strain limited tests were carried out at all the strain rates and the temperature rises were estimated from the area under the stress strain curves. X-ray diffraction was used to extract interatomic plane distances and crystallite dimensions. Differential Scanning Calorimetry (DSC) was employed to estimate the degree of crystallinity of the material and the kinetics of crystallisation. PET yield stress increased with strain rate with a sharp increase at rates of 10(3) s(-1) and above. It crystallised into the triclinic form at rates above 10(3) s(-1) beyond 140% strain but crystallisation was not observed at lower strain rates. Increases of up to 40% in crystallinity content were found which, it is concluded, were thermally induced after the test ended. The results shed tight on the development of crystallinity in PET as a function of strain, strain rate and temperature and indicate that the rapid increase in yield and flow stresses previously reported cannot be accounted for by increases in crystallinity.