The finest particle size fractions (less than or equal to 25 mum) in four fly ash samples front fluidised bed combustion of three biomass based fuels and a municipal solid waste fuel were size separated using a centrifugal SPLITT fractionation cell. The ashes were separated into different size fractions and the cadmium concentration, partition and speciation in each separated fraction were then investigated in relation to their possible leaching. The fractionation was evaluated by environmental scanning electron microscopy, which also provided indications of the associations between Cd and other main elements on the particles through the use of X-ray fluorescence mapping. The total concentration of Cd in each fraction was determined by AAS analysis which showed different dependences on the ash particle size in the case of biomass or waste fuels. In addition, the speciation of Cd in each fraction, investigated by sequential chemical extractions and X-ray powder diffraction analysis, showed Cd to be present mainly as non-easy leachable forms, i.e. oxide and silicates. A readily leachable fraction was found only in the municipal solid waste fly ashes. (C) 2002 Elsevier Science Ltd. All rights reserved.