The deformation and fracture behaviour of nylon-6/EPR (ethylene-propylene rubber) blends is studied as a function of strain rate and rubber content. Therefore, tensile experiments are conducted on notched specimens over a broad range of draw speeds (including strain rates as encountered in normal tensile tests and in impact tests). The blends with a high rubber content show super-tough behaviour at low and high speeds. In the intermediate-speed regime the fracture energy reaches a minimum (with a level comparable with that of unmodified nylon). The blends with a low rubber content show a transition from tough to brittle behaviour with increasing strain rate. The sudden rise in fracture energy with rising strain rate is believed to be caused by a transition from isothermal to adiabatic deformation. In the adiabatic regime the rise in temperature is high enough to melt the material around the crack tip. This thermal blunting mechanism causes crack propagation to be stable at high strain rates. This mechanism is affirmed by scanning electron microscope studies of the deformation zone.