A styrene-acrylonitril copolymer (SAN) was toughened by SAN-grafted polybutadiene core-shell rubber particles. Notched tensile specimens were fractured with a tensile speed ranging from 10(-4) to 10 m s(-1). The deformation processes close to the fracture surface were studied by means of transmission electron microscopy. A marked difference in the structure of the deformation zone was observed between low speed (10(-3) m s(-1)) and high speed (greater than or equal to 1 m s(-1)) deformed samples. At low tensile speed the structure of the deformation zone correlated closely with fracture mechanics theory. When the tensile speed was increased the deformation zone had a layered structure. In the zone 400-1.5 mu m below the fracture surface the deformation structure was similar to that at low speed. In the layer 1.5-0.5 mu m from the fracture surface the rubber particles were strongly deformed, but no cavities or crazes could be observed : Directly next to the fracture surface the high speed deformation zone showed a small layer (0.5 mu m) where all the deformation had vanished. it is suggested that due to high strain-rate plasticity at the crack tip a temperature rise occurs which is high enough to cause complete relaxation of the deformation in this layer. Therefore, locally the glass transition temperature of the matrix material was reached. The interaction between thermal effects and deformation processes at the crack tip is discussed.