The interface instability of the coextrusion flow of a polyethylene and a polystyrene is studied both experimentally and theoretically in a slit geometry. For prototype industrial conditions, we have found a stable/unstable transition which bounds the occurrence of stable/unstable sheets at die exit. By investigating a large range of processing conditions, we have shown that this transition is controlled by both temperature and flow rate ratios. Close to the transition, we used a transparent die to measure spatial amplification of different controlled perturbations at die inlet and pointed out the convective nature of the instability which exhibits a dominant mode (for which the instability is the most severe). We have then found that a convective stability analysis, using the White-Metzner constitutive equation, is able to account for the spatial amplification rate experimentally measured on controlled perturbation experiments. By considering that the instability is controlled by its dominant mode, we performed a convective stability analysis for all studied prototype industrial conditions and showed that such an analysis is able to forecast the occurrence of defects at die exit. (C) 2004 Elsevier B.V. All rights reserved.