This paper presents experimental results and numerical simulations for mono, blended and polydisperse polstryrenes of different molecular weights flowing within a slit geometry. Flow experiments were carried out on small (less than 10 g) quantities of polymer using a multi-pass rheometer and flow-induced birefringence images were obtained for well-defined flow boundary conditions. Experimental flow birefringence observations illustrate the similarities and differences in the flow behaviour between monodisperse and polydisperse polystyrene. For the case of monodisperse polystyrene a transition from "near-Newtonian" stress patterns for low molecular weight polystyrenes, to a highly unstable flow at high molecular weight was observed. Both blending and polydispersity enabled stable flows to be achieved at high flowrates. Experimental flow birefringence results and some pressure difference predictions were compared with numerical predictions. Two different computational approaches were followed, one using a viscoelastic integral K-BKZ/Wagner model within the finite element method solver Polyflow, and the other using the tube theory-based Pom-Pom constitutive equation and Lagrangian-Eulerian code flowSolve. Both numerical methods were able to capture certain experimental observations reasonably well in the stable flow regime, but were not able to predict the onset of the experimentally observed flow instabilities. (c) 2005 Elsevier B.V. All rights reserved.