In situ straining on a transmission electron microscope (TEM) stage has been used to study deformation mechanisms in a blend of poly(styrene-co-acrylonitrile) (PSAN) with PMMA/acrylate-rubber/PMMA core-shell particles, over a range of temperatures. Thin sections were tested at - 20, 23 and 60 degrees C at a constant tensile strain rate of 0.05% s(-1) Cavitation was observed at all three temperatures. At - 20 degrees C, the main deformation mechanisms were crazing of the PSAN matrix and fibrillation of the acrylate rubber. At 23 degrees C, crazing and shear yielding of the PSAN occurred simultaneously, with more extensive fibrillation of the rubber particles and drawing of material from the PMMA cores. This disruption of thermoplastic core material indicates that high stresses are generated within the modifier particles. At 60 degrees C, crazing could no longer be detected : shear yielding of the matrix and cavitation of the rubber particles were the main mechanisms of deformation.