A semiempirical model has been developed for the prediction of the grade efficiencies of a gas-solid cyclone for circulating fluidized bed (CFB) boilers. The model is based on the equilibrium orbit theory. Experiments were conducted in a large cyclone to gather data on grade efficiencies for model verification. The model gives a good agreement between predicted and experimental values of grade efficiencies. (C) 2004 Elsevier B.V. All rights reserved.