Toughening amplification of the neat poly(vinyl chloride) (PVC) and its reinforced version containing 25 phr of the cellulose acetate (CA)-compatibilized starch using methyl methacrylate-butadiene-styrene (MBS) core–shell particles was studied. The room temperature measured impact strength of the PVC showed mild increase up to 10 wt % with the addition of MBS particles. Then, the toughness enhanced discontinuously to super-tough plateau regime. The room temperature measured impact strength of PVC containing 20 phr of MBS particles, however, was reduced by as much as 95% when it was filled with 25 phr of the CA-compatibilized starch. In addition, the brittle–ductile transition (BDT) of the toughened PVC increased from 0 to 60 °C because of its reinforcement, even though the matrix number density of the core/shell particles remained almost constant. The decline in the impact strength and the rise in the BDT of the hybrid PVC system were attributed to the decrease in the shear deformable matrix and shear deformation propagation rate despite the increase in the process zone size. Maximum impact strength of the hybrid system at 60 °C (its BDT) increased to about 25% of the toughened PVC at its BDT (0 °C). The toughness amplification correlation of the toughened and hybrid PVC systems with their process zones fractional stress volumes under the impact load showed three regimes: quasi-tough, transition, and super-tough, which were superimposable on literature data regarding hybrid nylon 66 systems. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010