Developing centerline velocity and reattachment lengths were measured for viscoelastic polymer solutions in an abrupt 2‐to‐1 expansion. Test fluids were characterized for shear stress and first normal stress difference using a Weissenberg Rheogoniometer. The deviation from inelastic behavior is examined for0.31⩽n⩽0.58,2.7⩽Re⩽116.0and0⩽We⩽0.195.No deviation from inelastic behavior was observed forWe⩽0.047,while significant predevelopment of the flow field relative to inelastic behavior was observed for0.047⩽We⩽0.114.A flow instability was observed forWe⩾0.12.The conditions for unstable flow are compared to the reversed contraction flow and to criteria suggested in the literature.