Improving the impact resistance of plastics is a key to many applications. Today, dispersing rubber and inorganic particles into semicrystalline polymers is widely used to increase their impact strength without greatly altering other interesting properties such as elastic modulus or chemical resistance. Yet, the underlying mechanisms controlling such toughening are controversial. Hitherto it has been often suggested that a critical distance between particles which controls the brittle-to-tough transition is an intrinsic property of the polymer. On the contrary, we demonstrate here that differences in crystalline organization of the matrix can induce dramatic changes in toughening efficiency. A thermal treatment and microscopic observations strongly suggest that crystalline orientation, size of crystalline grains and molecular organization at grain boundaries play a determinant role in the toughening mechanisms. These observations may have important implications for designing and manufacturing tough plastic materials. (c) 2005 Elsevier Ltd. All rights reserved.