This paper presents selectivity curves based on experimental data obtained for four different groups of hydrocyclones. The results show that the effect of pressure on the shape of the selectivity curve is significantly smaller than that of the feed concentration and the ratio between vortex finder and apex size. The role of particle-particle interactions as a responsible mechanism for the fish-hook effect will be discussed. These interactions manifest themselves through the interstitial fluid, by secondary current and pressure field changes in the carrying fluid, caused by the presence of other particles. As a result, fines are dragged by the wakes generated behind larger particles. (C) 2002 Elsevier Science B.V. All rights reserved.