A comparative fundamental study was performed regarding the notched impact toughening performance of various rubber modified semicrystalline polyethylene terephthalate (PET) systems. Various modifiers with and without functional groups were evaluated: ethylene-co-propylene rubber (EPR), maleic anhydride grafted EPR (EPR-g-MA), glycidyl methacrylate grafted EPR (EPR-g-GMAx) and ethylene-glycidyl methacrylate copolymers (E-GMAx). Both binary and ternary blends (consisting of a preblend of EPR and a functionalised modifier) were examined. The most effective toughening route for PET is provided by dispersing a preblend of EPR and a low amount of E-GMAx. A minimum dispersed phase concentration of 30 wt% is needed to obtain a pronounced improvement of the impact strength and to induce a brittle-ductile transition of the fracture mode. The impact behaviour of the rubber toughened PET is primarily controlled by the morphological characteristics, i.e. the interparticle distance. An equal critical interparticle distance (IDc) of 0.1 mum was established experimentally for the different GMA compatibilised systems. This IDc is found to be independent of the amount of GMA functionalities present, the way of incorporation in the chain (grafting or copolymerisation) and the nature of the compatibiliser. The ternary PET/(EPR/E-GMA8) blends provided the best ultimate mechanical properties, displaying highly (15-fold) increased impact strengths and reasonable elongations at break. (C) 2002 Elsevier Science Ltd. All rights reserved.