The following study forms part of a programme of work designed to produce a commercial moulding material based upon Polyethylene terephthalate. The work described relates specifically to the investigation of the impact characteristics of PET, and improvements that may be obtained in the impact strength by incorporation of the rubbery phase into the glassy PET matrix. The two phase systems studied in greatest detail were PET/Ionomer (Surlyn A) blends. A grafting reaction that was found to occur in the melt between these two polymers has been investigated in some detail using a Weissenberg Rheogoniometer. Also reported are studies of the crazing properties and nature of the craze structures produced in amorphous PET, together with a study of the crack velocity profiles obtained during the high velocity impact fracture of PET mouldings. This latter work led to a correlation of the topographical features observed on the fracture surface with the crack velocity. As a result a stick-slip mechanism of crack propagation was proposed, involving craze formation and fracture.