PB-g-MMA core-shell impact modifiers were synthesized by seed emulsion polymerization and impact-modified PMMA/PVC blends were prepared by melt blending PMMA, PVC and PB-g-MMA at 160 degrees C. The PB-g-MMA particles were dispersed uniformly in the PMMA/PVC matrix. PMMA/PVC blends were prepared in the blend ratio from 100/0 to 0/100 and the rubber content was kept 16% in all the compositions. The effects of matrix composition on the mechanical properties and morphology of the blends were studied. It was found that when the matrix was a PMMA-rich system, the sample broke in a brittle mode and crazing of the matrix was the main mechanisms of deformation. When the matrix was a PVC-rich system, the sample broke in a ductile mode and the main deformation mechanisms were cavitation of the particle and shear yielding of the matrix. There existed a transition from crazing to shear yielding in the rubber-modified PMMA/PVC blend as the matrix composition varied.