Small diameter hydrocyclones have had an increasing use in performing difficult phase separations in many industries. Further, these 10 mm diameter hydrocyclones also have the potential to be applied to fine particle classification, in particular to collect the sub-micron fraction. The key operating parameters that affect the dewatering and classification performance of 10 mm. hydrocyclones are the operating pressure and feed solids concentration, both of which have previously been studied in detail. Previous studies on larger cyclones have modified the viscosity by temperature, solids concentration and sucrose addition. In industrial operations, the role of temperature may be significant, either due to climatic variations, or by active manipulation. In particular, the effect of temperature on the relationship between the water recovery and the bypass fraction has not been described. This is of importance, as fine classification requires both a small bypass and cutsize, while dewatering would benefit from a large bypass, small cutsize and, if possible, a low water recovery. In the present study, the effect of temperature between 10 and 60 degreesC was studied on low concentration suspensions of silica flour in water using a 10 mm diameter hydrocyclone. It was observed that the phase separation improves for increasing temperatures. The cutsize was decreased from 5.3 to 2.8 mum, while the water recovery decreased simultaneously. The bypass fraction, however, also increased significantly (from 0.36 to 0.48) with temperature, increasing the solids recovery and, importantly, did not follow the same trend as the water recovery. This has not been reported previously. The corrected cutsize changes were successfully predicted using the equilibrium orbit theory, while the solids recovery was interpreted using residence time relationships. These described satisfactorily the relationship with liquid viscosity changes due to temperature. (C) 2004 Published by Elsevier Ltd.