The interfacial stability of superposed flow of polypropylene (PP) and high-density polyethylene (HDPE) in a converging-diverging channel geometry is experimentally investigated. Using the apparatus and procedures developed in our previous study [20], we have investigated the role of base flow kinematics on the interfacial stability of the HDPE/PP system. A comparison of experimentally determined neutral stability contours and disturbance growth rates in parallel, converging and diverging channel flows has revealed that channel convergence tends to stabilize interfacial disturbances while channel divergence has a destabilizing influence. Moreover, we have shown that the most dangerous wave number when made dimensionless with respect to the thickness of the more elastic and/or viscous component is of 0(1). In addition, based on a simple analysis, we have shown that the mechanism of interfacial instability in two-layer viscoelastic flows is due to the coupling between the perturbation velocity field and the polymeric stress gradient in the base flow.