In many cases, finest particles in hydrocyclones present higher separation efficiencies than can be expected when considering the theory of single particle sedimentation. The analysis of sedimentation of polydisperse particle systems shows that particle movement is influenced remarkably by solids concentration and particle size distribution. Sedimentation experiments with a special centrifuge confirm the action of combined effects, of particle entrainment, of countercurrent flow of displaced water and of elevated values of pulp density and viscosity due to the disperse solid phase. These influences can be summarized in a sedimentation model which allows better calculation of the separation result of the hydrocyclone. As a consequence, the typical course of the separation curve of the hydrocyclone indicates a more or less marked so-called fish-hook in the fine particle range. (C) 2004 Published by Elsevier Ltd.