Visualizations of the 3-D flow in a 4:1 square-square sudden contraction for two viscoelastic Boger fluids and two Newtonian fluids were carried out at low Reynolds numbers. In these creeping flow conditions, the vortex length remained unchanged for Newtonian fluids, whereas a nonmonotonic variation with flow rate was observed for the Boger fluids. Initially, the corner vortex slightly increased with flow rate to a local peak at a Deborah number of De(2) approximate to 6, before decreasing significantly to a minimum at De(2) approximate to 15 (De(2) is based on downstream characteristics). Finally, for Deborah numbers > 20 there was intense vortex enhancement until a periodic flow was established at higher flow rates (De(2) approximate to 45-52). The strong elastic vortex enhancement was preceded by the appearance of diverging streamlines on the approach flow and, for the Boger fluid with higher polymer concentration, vortex enhancement took place through a lip vortex mechanism. (c) 2005 American Institute of Chemical Engineers.