Real-time stress-whitening experiments were performed on initially transparent poly(vinyl chloride) (PVC)/methyl methacrylate-butadiene-styrene graft copolymer (MBS) blends containing very small MBS rubber particles. The chemical composition of the different MBS rubber particles was held constant and it was found that the onset of internal rubber cavitation in the blends during a tensile test depends only on the size of the rubber particles. The real-time, stress-whitening experiments indicate that the resistance against cavitation increases with decreasing rubber particle size. Small particles are unable to cavitate. This is further confirmed independently on the basis of transmission electron microscopy observations and density measurements on the deformed samples.