The flow of a polystyrene in an abrupt slit contraction with transparent side walls has been characterized using both flow induced birefringence (FIB) and laser-Doppler velocimetry (LDV), under stable and unstable flow conditions. Under steady-state flow conditions, at the contraction entry recirculating vortices are observed, with velocities depending on flow conditions. When the flow rate is increased, periodic oscillations with the same frequency are observed for FIB patterns and the velocity field, independent of the flow rate. The oscillations correspond to periodic volume instabilities of the extruded strands. For higher flow rates, entry vortices destabilization leads to a sudden discharging of these vortices into the main flow, inducing chaotic defects in the extrudate. (C) 2007 Elsevier B.V. All rights reserved.