The use of structural core/shell latex particles as toughening agents provides a model system which allows independent control of several key factors that influence the fracture toughness of modified plastics. This paper focuses on varying the shell composition of poly(butadiene-co-styrene) [P(B-S)] core/poly(methyl methacrylate) (PPMA) shell particles by incorporating acrylonitrile (AN) comonomer into the PMMA shell at various AN/MMA ratios and by crosslinking of the shell at various AN/MMA ratios. It was found that the degree of particle dispersability in the epoxy matrix can be precisely controlled by the AN content in the PMMA shell and by crosslinking the PMMA in the shell. It was also found that the degree of particle dispersability plays a crucial role on the fracture toughness of the modified epoxies. A microclustered morphology provides a much higher toughness than a uniform particle distribution.