This paper presents an experimental study on the formation of a polymer flow instability seen in a range of polystyrene melts flowing through contraction-expansion slit geometries. Seen for both monodisperse and polydisperse materials, it took the form of an oscillation in the principal stress difference (PSD) and velocity field perpendicular to the bulk flow. The instability originated downstream of the slit and overtime propagated back upstream such that both regions were unstable. For the monodisperse materials, flowrates were characterised using a theory for linear entangled polymers. This allowed a comparison of the flow regime, with respect to both chain stretch and orientation, and the onset of the instabilities. These are presented as stability maps for comparison with future numerical work. In all cases, no single flowrate, Weissenberg number or critical pressure drop was seen which triggered the instability. The variation of slit curvature and length was found to affect the onset of the flow instability, but not its general form. For all materials, an increase in polydispersity and decrease in molecular weight led to increased processability of the material, delaying the onset of the instability until higher flowrates. (C) 2008 Elsevier B.V. All rights reserved.