Fatigue crack growth measurements have been made on blends of polycarbonate (PC), poly(methyl methacrylate) (PMMA), poly(styrene-acrylonitrile) (PSAN) and poly(vinyl chloride) (PVC) with two impact modifiers. In one modifier, the particles have a polybutadiene core and a PMMA shell; in the other, there is a PMMA core, a poly(butyl acrylate-styrene) inner shell and a PMMA outer shell. The overall pattern of fatigue crack growth is similar in all eight blends, but there are significant quantitative differences in ΔKth, the threshold value of the cyclic range of stress intensity factor, and in the growth rate characteristics above ΔKth. These are related to various features seen on the fracture surfaces under the scanning electron microscope, and hence to the mechanisms of deformation and fracture near the crack tip, which include crazing, particle–matrix debonding, and ductile drawing.