The objective of this work is to characterize the parameters affecting the particle concentration distribution and flow field of a concentrated suspension undergoing steady flow in an abrupt axisymmetric 1:4 expansion. Of particular interest are the relationships between imposed operating conditions and the resulting spatial particle distribution and the interaction between particles and recirculating flow regions. Experiments were conducted to determine the effect of the bulk particle volume fraction, tube-particle radius ratio, and Reynolds number on observed concentration and flow patterns. Particle concentration and velocity profiles were measured by using nuclear magnetic resonance imaging. Results indicate that inlet concentration profiles formed in the upstream narrow tube greatly influenced behavior downstream, more so than direct interparticle collisions in the abrupt expansion. Also, particle depletion in recirculating flow regions was observed under all conditions studied and is consistent with the results of previous work expressed in terms of the tube-particle radius ratio. Finally, in all cases, the lengths of recirculating regions were greater for suspensions than for Newtonian fluids at equivalent Reynolds numbers, and increased with bulk particle volume fraction. The flow behavior is reminiscent of shear thinning in a single-phase material, but shear-induced particle migration is a much stronger effect. (c) 2005 The Society of Rheology.