The laminar flow of a nonlinear viscoplastic fluid through an aimular sudden expansion was investigated experimentally. In addition to a yield-stress shear-thirming fluid, the flow of a Newtonian fluid through the same expansion was also studied to form a baseline for comparison. Velocity vectors were measured on the vertical center plane using digital particle image velocimetry (PIV). From these measurements, two-dimensional contours of axial and radial velocity as well as the stream function were calculated covering the separated, reattached and redeveloping flow regions. The Reynolds number range was Re=1.8 to 31, based on the upstream pipe diameter and bulk velocity, and the corresponding yield number range for the non-Newtonian fluids was Y=0.28 to 1.22. The results obtained for a power-law index of n=0.68 indicate the existence of two distinct flow regimes as the Reynolds number increases and the yield number decreases. For a combination of low Reynolds numbers and high yield numbers, a non-moving fluid zone is observed immediately downstream of the step and no separated flow zone exists. For the higher Reynolds number and the lower yield number flow, a separated flow zone does exists downstream of the expansion step, which is followed by flow reattachment characterized by a three-dimensional zone of stagnant fluid.