Although several mathematical models describing hydrocyclone performance exist in the literature, only a few suitably describe classification of non-Newtonian slurries in small-diameter hydrocyclones (smaller than 75 mm of diameter). In order to develop a mathematical model accounting for the effects of slurry rheology on classification, experiments have been conducted with phosphate ore slurries of variable solids concentration and chemical environment, covering a wide range of rheological conditions, both Newtonian and non-Newtonian. Semi-empirical models for estimating the hydrocyclone capacity and corrected cut size and an empirical model of water split have been proposed and/or modified from the literature in order to describe classification under such extreme rheological conditions. Of particular interest is the application of a model developed by the authors and based on the residence time theory, that is used to predict the corrected cutpoint. Experimental results demonstrated that, under the conditions studied, hydrocyclone capacity and sharpness of separation were not affected by slurry rheology. It was also found that the plastic viscosity is the most significant parameter for modeling the corrected cut size in 50 and 25 mm diameter hydrocyclones. (C) 2002 Elsevier Science Ltd. All rights reserved.