An empirical model of a 254mm water-only cyclone treating minus 17001im coal has been developed. The model includes the prediction of by- passing, SG50 corrected, water split and the use of a single equation for the reduced performance curve of particles in the range 1700-106pm. Particles finer than 106pm did not separate according to specific gravity. Simulations using the model showed the water-only cyclone performance to be dependent more upon feed size distribution than washability when the feed contains high-ash fines, but that, with a deslimed feed, the yield and ash content of the over- flow should be controllable by altering feed rate or feed pulp density.