The characterization of solid wastes from full-scale circulating fluidized bed combustors (CFBC) is necessary to ensure that disposal procedures or utilization strategies for the waste solids are successful. Pilot plants are extremely useful in providing hydrodynamic heat and mass transfer data that can be used to design and predict the performance of larger units. Combustion studies indicate that data from pilot-scale units can be used to approximate the behavior of a full-scale plant for different fuels and operating conditions, even when the pilot plant is not designed to properly scale the commercial unit. However, the same does not seem to be true for the determination of reduced sulphur, the other is species and geotechnical or physical properties of the solid wastes generated from pilot plants. The results of analyses of samples generated from two units are discussed. One is a 150 by 150 mm square, 7.3 m high pilot-scale CFBC located at the University of British Columbia and 22 MWe CFBC located at Chatham, New Brunswick. This unit is operated by the New Brunswick Electric Power Commission (NBEPC). Both used the same New Brunswick coal containing 7 percent sulphur. The data presented indicate that the pilot-scale unit can significantly overpredict the formation of sulphides, and compared with the full-scale unit, produces residues with much less promise for either disposal or utilization in low-strength concretes. The results strongly suggest that further work is necessary to understand better the phenomena that produce sulphides and affect the geotechnical properties of wastes.