A model is proposed to explain the observed relationships between particle size and fracture resistance in high-performance blends, which typically reach maximum toughness at particle diameters of 0.2-0.4 mu m. To date there has been no satisfactory explanation for the ductile-brittle (DB) transition at large particle sizes. The model is based on a recently developed criterion for craze initiation, which treats large cavitated rubber particles as craze-initiating Griffith flaws. Using this criterion in conjunction with Westergaard's equations, it is possible to map the spread from the notch tip of three deformation mechanisms: rubber particle cavitation, multiple crazing and shear yielding. Comparison of zone sizes leads to the conclusion that maximum toughness is achieved when the particles are large enough to cavitate a long way ahead of a notch or crack tip, but not so large that they initiate unstable crazes and thus reduce fracture resistance. (C) 2009 Elsevier Ltd. All rights reserved.