The microvoiding mechanisms and mechanical properties of a blend of poly(vinyl chloride) (PVC) with methyl methacrylate-butadiene-styrene (MBS) graft copolymers are investigated. It is found th at the toughening effect is independent of the type of microvoiding mechanism occurring in the blend. Both internal cavitation of the MBS particles and debonding at the PVC/MBS interface relieve the triaxial tension and thereby promote shear yielding of the PVC matrix.