Stress-strain behavior coupled with fractography was used to investigate the weld line strength of 30/70 w/w poly(vinyl chloride)/high density polyethylene (PVC/HDPE) blends. The weld line strength determined in uniaxial tension depended upon the domain shape of the PVC phase at the weld line, with elongated domains causing weld line weakness. To alter the PVC domain shape, the viscosity ratio (eta(PVC)/eta(HDPE)) Of the blend was varied by changing the PVC molecular weight. The domain shape at the fracture initiation site was used in conjunction with a modified Nielsen approach to predict the ductile to brittle transition at the weld line. For the composition studied, a critical aspect ratio of the PVC phase of 1.24 was determined. Elongation of the PVC domains above this critical value led to brittle weld lines. The domain shape at the weld line was modeled by a Taylor analysis of droplet deformation in an elongational flow field. The calculations predicted that a viscosity ratio of 21 would produce a particle with an aspect ratio of 1.24. The observed weld line strength confirmed this prediction: blends with a viscosity ratio below 21 were brittle. and those with a viscosity ratio above 21 had ductile weld Lines. (C) 2000 Elsevier Science Ltd. All rights reserved.