This study is a fractographic examination of epoxy resin modified with scrap car tyre regrinds used as toughening phase. Five different surface treatment techniques, used to improve the compatibility at the interface between the rubber particles and the epoxy matrix, were: oxygen plasma, two silane coupling agents, acrylic acid and acrylic acid/benzoylperoxide mixture. Rubber parts of ground scrap car tyres with a size range of 355-500 mum after surface treatment were mixed with epoxy. This mixture was poured into molds to obtain fracture toughness test specimens. After curing, plane strain fracture toughness tests were performed and the fractured surfaces of the specimens were examined under the scanning electron microscope. Fractographic studies indicated that the increase in fracture toughness values were due to the improved interface leading to crack deflection as the main rubber toughening mechanism. Secondary rubber toughening mechanisms observed were shear deformation and some debonding initiated at the interface. (C) 2001 Elsevier Science Ltd. All rights reserved.