The toughening mechanisms and their sequence in rubber modified plastics is determined experimentally through the use of a multi-axial testing combined with and a rubber-modified material with favorable properties. The multi-axial testing was achieved by internally pressurizing and uniaxially loading hollow cylinders. This test allows independent control of the mean stress (volume change) and octahedral shear stress (shape change) between regions of uniaxial compression and equal biaxial tension. The rubber-modified material consists of a methacrylate-butadiene-styrene (MBS) core-shell modifier dispersed in polyvinylchloride (PVC). The whitening behavior of this material during deformation is favorable, since it provides optical verification of cavitation or debonding. In situ light transmission measurements of the MBS modified PVC in the multi-axial stress state test allows the mechanics associated with rubber particle cavitation to be evaluated. The onset of whitening in these systems occurs at a constant octahedral shear stress under a positive mean stress prior to yielding. This behavior is in disagreement with recent models, which predict cavitation occurring at a constant positive mean stress in MBS modified PVC.