The motion of 18F radioactively labeled papermaking fibers flowing through an axisymmetric 1:5 sudden expansion has been studied using positron emission tomography (PET). Various length fractions of a mechanical pulp were radioactively labeled and then introduced into a nonradioactive aqueous 0.4 wt. % (consistency) wood pulp suspension. Fully three-dimensional (3-D) images were reconstructed both upstream and downstream of the expansion plane for cases in which the upstream velocity U was set from 0.5 to 0.9 m/s (an approximate Reynolds number range of 7,000 to 13,000). Two distinct flow regimes were clearly identified. With U ∼ 0.5 m/s we find that the fiber suspension was not fluidized, and the tracer fibers passed through the expansion as a plug. No mixing was observed between the confined central jet and the static outer region. At larger velocities, we observed that the papermaking suspension was fully fluidized. Our results in this regime indicate that albeit fluidized, concentration inhomogeneities were evident. We find that a particle depletion zone was evident between the central jet, and the recirculating zone resulting from the inlet concentration profile formed in the upstream tube. Particle accumulation was observed in the vortices. No significant differences were observed between the different length tracer fibers. © 2007 American Institute of Chemical Engineers.