Improved understanding of the behaviour of floes within a hydrocyclone is necessary to progress the use of hydrocyclones for the clarification and thickening of fine particles. This paper describes experimental and modelling work to investigate the separation behaviour of flocculated particles in a hydrocyclone to better understand the different mechanisms influencing separation. Flocculated pseudo-monodisperse and polydisperse alumina trihydrate in a 1 wt.% water slurry was separated in a 22-mm Mozley hydrocyclone. Floc structure properties, floc size distribution and also primary particle composition within those floes were measured experimentally for all flows (feed, overflow and underflow). For the case study system at 100-kPa hydrocyclone-operating pressure, there was an improvement in reduced efficiency from 0.75 to 0.84 with flocculation. Contrary to the assumption of literature models, breakage was limited, with the effect of floc hydrodynamics determining the separation behaviour. With density being the most important hydrodynamic effect, the reduced density of floes compared to primary particles is the main reason for the limited improvement flocculation could achieve in hydrocyclone separation and not, as often suggested, floc break-up. A micro-scale semiempirical model was developed to predict the separation performance of flocculated particles in hydrocyclones. The model represents the relationships between floc hydrodynamics and hydrocyclone classification as well as between floc strength and hydrocyclone shear. Comparison with the experimental results is used to highlight the areas where further work is required to progress understanding. (C) 2004 Elsevier B.V. All rights reserved.