The influence of interfacial adhesion on the tensile properties, including toughness, of poly(vinyl chloride) (PVC)-nitrile rubber (NBR) blends with the morphology of well-dispersed rubber particles has been investigated using two types of blends. The first type, which contains NBR26 (NBR with 26 wt% acrylonitrile (AN)), has a higher interfacial adhesion strength than the second type that contains NBR18. The secant modulus and yield stress of the blends were found to be independent of interfacial adhesion. On the other hand, the elongation-at-break and toughness (defined as the area under the stress-strain curve to break) depend strongly on the interfacial strength. The effects of rubber particle size, size distribution and rubber volume fraction on the tensile properties have been combined into the effect of a single morphological parameter, the matrix ligament thickness T. Both the elongation-at-break and toughness increase as T decreases. At T < 0.06 m the blends with stronger interfacial adhesion (PVC-NBR26) have much higher elongation-at-break and toughness. Stress whitening was observed in all deformed PVC-NBR18 blends. For PVC-NBR26 blends, however, stress whitening occurred only at T > 0.06 mum Transmission electron microscopy studies revealed that debonding at the PVC-NBR interface is the sole microvoiding mechanism that causes stress whitening.