A feed of particles covering a wide range of particle sizes and densities was placed into a novel separator, referred to as the Reflux Classifier, and fractionated at a series of fluidization velocities. Double fractionation was then achieved when each product was sieved into narrow size fractions. For the coal particles used in this study, the ash % was determined for each of the samples. The ash % correlated strongly with particle density, and hence was a useful proxy for the particle density. A new Fractionation Algorithm was developed, which provided a method for estimating the feed size and density distribution from the Double Fractionation data. The cumulative yield % versus the cumulative ash % curves for each size fraction, and for the overall feed were then determined. The findings from the study were validated against accurate results obtained using the sink-float method. Excellent agreement with the curves obtained using the sink-float method was obtained. The sink-float method requires the use of heavy liquids; these are invariably toxic and expensive, hence a viable alternative is needed. The water based fractionation method, in conjunction with the Fractionation Algorithm, may prove to be a suitable substitute for the sink-float method. Precise details of the method and algorithm are therefore provided in order to permit the adoption of this new approach by other laboratories. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.