We have derived a new equation for correlating the average matrix ligament thickness with other morphological parameters of a polymer blend with the pseudonetwork morphology in order to evaluate the effect of rubber particle spatial distribution on the brittle ductile transition of poly(vinyl chloride) (PVC)/nitrile rubber (NBR) blends. Two types of rubber particle spatial distributions, i.e. well-dispersed particles and pseudonetwork, are investigated. The formation of PVC primary particles favours the toughening of PVC. An approximate master curve of brittle ductile transition for the blends with the pseudonetwork morphology is obtained by plotting the impact strength, versus the average matrix ligament thickness. The critical matrix ligament thickness for the blends with the pseudonetwork morphology is 0.11 mu m, which is much larger than that (0.059 mu m) for the blends with the morphology of well-dispersed rubber particles. Therefore, pseudonetwork morphology is much more effective in toughening PVC than the morphology of well-dispersed rubber particles. Moreover, the critical matrix ligament thickness depends on the rubber particle spatial distribution, and is thus not a characteristic of the PVC matrix. The effects of morphological parameters and PVC molecular weight on the toughness of PVC/NBR blends are discussed.