Toughening mechanisms in blends of isotactic polypropylene and Noryl polyphenylene oxide/polystyrene (iPP/Noryl) are studied using optical microscopy, scanning electron microscopy and transmission electron microscopy. Large Noryl particles (10-15 mu m) are formed in iPP/Noryl blend and crazing is found to be the dominant toughening mechanism. A detailed investigation of fracture mechanisms reveals that Noryl particles help trigger and stabilize massive crazes in the iPP matrix. Incorporation of a small amount of styrene-ethylene-propylene (SEP) compatibilizer helps reduce Noryl particle size and improve interfacial adhesion between iPP and Noryl particles. Crazing and shear banding mechanisms are found to operate sequentially in iPP/Noryl/SEP blends. As a result, significantly improved toughness is obtained.