Poly(butylene terephthalate), PET, can be impact modified by blending with appropriate ABS materials. The effect of ABS type and processing conditions on the notched Izod impact strength of PET blends is examined in depth. Of three emulsion-made ABS materials containing 38 to 50% rubber, the one with the highest melt viscosity (50% rubber and a broad rubber particle distribution) proved the least effective for improving the impact strength of PET when processed at high temperatures (260 degrees C). Blends prepared in twin screw vs a single screw extruder have similar impact behavior. Melt temperatures greater than 240 degrees C used during molding have little effect on the crystalline behaviour of PBT; however, they reduce the effectiveness of some ABS grades for impact modification. Blends moulded at higher temperatures show a coarse phase morphology with large, poorly dispersed ABS domains. PBT/ABS blends can have excellent mechanical properties in the absence of any compatibilizer; however, a compatabilizer would no doubt improve the stability of the morphology of the blends and may lessen their dependence on process conditions.