This report covers experimental studies and numerical modelling of interfacial instability in the bi-layer coextrusion flow of two low-density polyethylene melts. Melt streams are converged at an angle of 30 to a common die land. Melt stream confluence was observed in two coextrusion die arrangements. In one die design, which we term 'bifurcated' the melt stream is split by a divider plate in the die after being delivered from a single extruder. In the other design melt streams are delivered to a die from two separate extruders. In each die design melt flow in the confluent region and die land to the die exit was observed through side windows of a visualization cell. Velocity ratios of the two melt streams were varied and layer thickness ratios producing wave type interfacial instability determined for each melt for a variety of flow conditions. Stress and velocity fields in the coextrusion arrangements were quantified using stress birefringence and particle image velocimetry techniques. Wave type interfacial instability occurred in the processing of the low-density polyethylene melts at specific, repeatable, stream layer ratios. The birefringent pattern in the confluence region and the beginning of the die land appeared stable even when the extrudate exhibited instability. However, disturbances were observed in the flow field near the exit of the die land. The study demonstrates conclusively it is possible for interfacial instability to occur in the coextrusion of the same melt. The study also shows that wave type interfacial instability in the coextrusion process is not caused by process perturbations of extruder screw rotation. Increased melt elasticity appears to promote this type of instability. A modified Leonov model and Flow 2000 (TM) software was used to simulate the LDPE melt flows through these geometries. There was reasonable agreement between modelled at experimentally determined stress fields. Modelling however provided far more detailed stress gradient information than could be resolved from the optical techniques. A total normal stress difference (TNSD) sign criterion was used to predict the critical layer ratio for the onset of the interfacial instability in one die arrangement and good agreement between theory and experiment has been obtained. (C) 2008 Elsevier B.V. All rights reserved.