The chemical characteristics of fly ash samples from combustion of three fuels: coal, peat and wood chips, have been studied. The ash materials were collected in the 12 MW CFB at Chalmers University of Technology during full load combustion with similar standard combustion parameters. The samples were characterized in terms of chemical composition, mineralogy and leaching behavior. The special characteristics of the mineral matter in each fuel were reflected in the leaching results. Upon reaction with moderate amounts of water secondary mineral phases such as ettringite and calcite, were formed. These compounds were shown to affect the leaching rates for calcium and sulfate as well as the pH of the leachates.