The effect of slurry viscosity on hydrocyclone classification was studied by installing an on-line viscometer set-up on a hydrocyclone test rig, This system used a vibrating sphere viscometer and a specially designed slurry presentation device to avoid settling of solids during viscosity measurement. Test samples were prepared from ground silica (80% passing 65 microns) and water, Both the solids content and temperature of the samples were varied to change the slurry viscosities. A modified version of Plitt＇s model was then used to predict the d(50)(c) size. Unlike the original model where slurry viscosity was inferred from solids concentration, the modified model incorporates a viscosity parameter from direct measurement. Therefore, the new modified model predicted the cut size precisely when the viscosities of the slurries were altered by means other than changes in the percent solids, In other work, it was found that the slurries-in our experiments were in the Newtonian flow regime, and their viscosities were independent of shear rate. As a result the measured apparent viscosity in this work was identical to the true viscosity. From this study it was found that d(50)(c) is proportional to the 0.35th power of the slurry viscosity. The water-to-underflow ratio (R(f)), also tended to increase with increasing viscosity, while the reduced efficiency curve was not affected.