New toughened poly(butylene terephthalate) (PBT) materials were obtained by melt blending with poly(ethylene-octene) copolymer (PEO) and maleic anhydride grafted PEO (gPEO) in a twin screw extruder followed by injection moulding at two injection speeds. The presence of either PEO or gPEO did not influence either the nature of the PBT phase or the crystallisation of PBT. Low injection speeds (7 cm(3)/s) and gPEO provided the best mechanical response. Increasing levels of maleic anhydride in gPEO led to a continuous overall decrease in the particle size, that was the most important when the particle size of the ungrafted PEO and that of the PEO at the minimum grafting level were compared. The decrease stopped at a grafting level between 1.14 and 1.80 due to the viscosity increase with the grafting level. Super-tough PBT based blends with impact strength more than twenty-fold that of PBT were obtained at PEO contents equal to or higher than 15%, and at decreasing PEO contents when the grafting level increased. The inter-particle distance (tau) is the parameter that controls toughness in these PBT/PEO blends. When the critical 7 (7,) measured in this work for PBT was compared with those obtained in other toughened blends at constant test conditions and rubber properties, 7, decreased as the modulus of elasticity of the matrix increased. When tau (c) of this work was compared with those obtained in other PBT/rubber blends, it depended on the modulus of elasticity of the dispersed phase, and it increased rather linearly as the PBT modulus to rubber modulus ratio increased.