The effects of phase morphology, interfacial adhesion and filler particle shape and volume fraction on the fracture toughness of polypropylene (PP) filled with CaCO3 or Mg(OH)(2) and ethylene-propylene elastomer (EPR) were investigated. Separation of the inorganic filler and elastomer particles was achieved using maleic-anhydride-grafted PP (MPP) to enhance the inorganic filler-matrix adhesion. Encapsulation of the rigid filler by the elastomer was achieved by using maleic-anhydride-grafted EPR (MEPR) to increase the inorganic filler-elastomer adhesion. The two limiting morphologies differed significantly in fracture toughness under impact loading at the same material composition. A model for a mixed mode of failure, accounting for the plane strain and plane stress contributions to the strain energy release rate, G(c), was used to predict the upper and lower limits for G(c) for the two limiting morphologies over an interval of elastomer volume fractions, v(e), from 0-0.2 at a constant filler volume fraction, V-f = 0.3, and over the filler volume fraction from 0-0.4 at constant EPR content. The role of material yield strength in controlling fracture toughness has been described successfully using Irwin’s analysis of plastic zone size. The presence of elastomer enhances both the critical strain energy release rate for crack initiation, G(c), and the resistance to crack propagation as expressed by Charpy notched impact strength for the two limiting morphologies. Satisfactory agreement was found between the experimental data and predictions of upper and lower G(c) limits.