The deformation mechanisms during fracture of a Nylon 6/ABS blend compatibilized with an imidized acrylic polymer were compared to those of an uncompatibilized blend. A postmortem examination of deformed zones in samples loaded to failure in a double-notch four-point-bend geometry was made using transmission electron microscopy (TEM). For the compatibilized blend, cavitation of the rubber particles followed by massive shear yielding of the polyamide matrix was concluded to be the sequence of events leading to toughness; while, for the brittle uncompatibilized blend, the evidence indicated that a lack of adhesion at the Nylon-ABS interface prevented the rubber particles from cavitating and the subsequent plastic deformation of the polyamide matrix. (c) 1994 John Wiley & Sons, Inc.