No / The effect of side stream channel height on flow stability in 30 degrees coextrusion geometries was investigated. The studies were conducted on a Dow LD150R low density polyethylene melt using a single extruder to feed a flow cell in which the delivered melt stream was split before, and rejoined after, a divider plate in a slit die. Wave type interfacial instability occurred at critical stream thickness ratios. Reducing the side stream channel height broadened the layer ratio operating range before the onset of interfacial instability, therefore improving process stability. Stress fields were quantified and used to validate principal stress differences of numerically modelled flow. Stress field features promoting interfacial instability in each of the die geometries were identified. Interfacial instability resulted when the stress gradient across the interface was asymmetric and accompanied by a non-monotonic decay in the stress along the interface from its inception.