Rubber toughening of a series of blends constituted by a polypropylene (PP) matrix added with talc, and modified by ethylene-propylene (EPR) and/or ethylene-butene rubber (EBR), with different molecular weights, was investigated. The fracture toughness was measured by an elasto-plastic fracture mechanics approach, applying the methodology of the essential work of fracture, and by conventional testing. It was found that : (i) EBR has a higher toughening efficiency than EPR; and (ii) the fracture toughness increases by increasing the molecular weight of the dispersed elastomeric phase. Measurements of volume change and temperature increase in specimens stretched during tensile tests indicated that the presence of EBR in the blends reduces cavitation and crazing, and induces a large amount of shear yielding. Some difference in the microcavitation mechanisms induced by each of the two elastomers was shown by electron microscopy analysis. An explanation of this behaviour on the basis of structural changes within the matrix or at the rubber-matrix interface was sought. Dynamic thermomechanical analysis indicated that EBR has a stronger interaction than EPR with the PP matrix.