Experimental observations of the flow through an axisymmetric 4:1 contraction for the shear-thinning test fluid S1 are presented. In order to provide the basis for comparison with numerical simulations of the flow, the linear viscoelastic spectrum and the nonlinear parameters for three constitutive models are also obtained, based on the rheological characterisation of the fluid by Ooi and Sridhar [J. Non-Newtonian Fluid Mech. 52 (1994) 121-135]. The Phen-Thien-Tanner model provides the best fit to the data, but a good fit can also be obtained for the Giesekus model if the fit to the extensional stress growth coefficient is restricted to low strains. The results of flow visualisation experiments are presented for the flow of S1 through a 4:1 axisymmetric contraction with either an abrupt entry or a rounded re-entrant corner. The dimensionless vortex size increased with Weissenberg number for each geometry, with a larger vortex observed for the abrupt entry geometry for identical flow conditions. No elastic lip vortex or flow instability was observed for We less than or equal to 5.0.