This study attempted to correlate morphological changes and physical properties for a high rubber content acrylonitrile-butadiene-styrene (ABS) and its diluted blends with a polystyrene-co-acrylonitrile) (SAN) copolymer. The results showed a close relationship between rubber content and fracture toughness for the blends. The change of morphology in ABS/SAN blends explains in part some deviations in fracture behavior observed in ductile-brittle transition temperature shifts. (C) 2004 Wiley Periodicals, Inc.