This paper discusses the use of polymeric surface active chemical agents for reducing the viscosity of dense medium suspensions. A laboratory-scale device was developed to measure shear stress at various shear rates, using a rotating bob and a circulating stream of heavy medium suspension. Ferrosilicon and magnetite suspensions were tested over a range of solid volume concentration and varying additions of clay, to simulate slimes build up. In most cases, a relatively modest addition of polymer reduced the viscosity of the medium by about 20% with up to 50% reduction achievable for some specific gravities and slimes levels. Reduction of medium viscosity could allow separation of materials at a finer size than is currently economically feasible, increase the efficiency of separations at higher particle sizes, or allow for cheaper media through the use of mixed magnetite/ferrosilicon suspensions. (C) 2004 Elsevier Ltd. All rights reserved.