Blends of polysulfone (PSF) and poly-phenylene sulfide) (PPS) exhibit ductile behavior, below 35% by weight PPS, under tensile loading conditions. However, the blends are notch sensitive to Izod impact. The use of a core-shell type rubber-modifier effectively toughens the blends. Notched Izod impact strength rises, from similar to 50 J/m to about 900 j/m, by increasing rubber content from 0% to 10-15%. It remains constant at a rubber content > 10-15%. Scanning electron microscopy (SEM) is used to study the morphology of the fracture surfaces. At low modifier content (5%), smooth or mesa-like fracture surfaces are observed. Voids and interfacial debonding are revealed. With a higher concentration of toughening agent (> 10%), some crazing is evident but not consistent. However, matrix yielding and extensive plastic flow of the PSF/PPS matrix are seen throughout, with a higher level of rubber modifier.