While it is recognized that the heterogeneous particles in HIPS play the dual role of providing multiple sites for craze initiation in the polystyrene (PS) matrix and allow the stabilization of the crazing process through cavitation/fibrillation in the PB phase within the particle, the precise role of particle morphology is not well understood or quantified. This work probes the micromechanics of uniaxial tensile deformation and failure in rubber-toughened PS through axi-symmetric finite element representative volume element (RVE) models that can guide the development of blends of optimal toughness. The RVE models reveal the effect on craze morphology and toughness by various factors such as particle compliance, particle morphology, particle fibrillation and particle volume fraction. The principal result of our study is that fibrillation/cavitation of PB domains within the heterogeneous particle provides the basic key ingredient to account for the micro- and macro-mechanics of HIPS. (C) 2009 Elsevier Ltd. All rights reserved.