The rheological behaviour of operating and consequently contaminated magnetite suspensions, such as those used in the dynamic dense media separation of coal is investigated. The apparatus and the procedures used for rheological characterization and viscosity measurement are described. Wide ranges of density and suspension contamination by coal fines are explored. A mathematical framework is presented, which allows the prediction, of the relative viscosity as a function of solids volume concentration, disperse phase composition and shear rate. The adjustable parameters of the suggested model are calculated on the basis of the experimental viscosity data collected on the tested magnetite dense media. The reliability, general applicability and usefulness of the proposed model are pointed out