Combustion residues generated from different incineration processes create major environmental problems because these materials contain high amounts of toxic substances. In this communication, we report the leaching behaviours of three residues along with their mineralogical compositions. Column leaching experiments at two different pH values were conducted to study the leaching behaviours. Leachants were analyzed to determine the pH, the concentrations of some toxic elements like Pb, Cr, As, Cd, Se, B, Mo and the volume of leachants passing through the columns. Raw and some treated residues were also characterized by XRD and SEM-EPMA techniques. We found that the concentrations of some toxic elements present in the residues as well as in the leachant fractions collected at different time intervals after leaching were very high. The amounts of soluble salts and toxic elements in the municipal solid waste incinerator (MSWI) ash were higher than those in the other two residues. The order of alkalinities of the three residues was: fly ash (FA) > MSWI ash > sewage sludge ash (SS ash). The final pH of the leachant was related to the volume of leachant passing through the column. Some leachant properties were related to the mineralogical compositions of the residues and to the secondary minerals formed during the leaching period. (c) 2005 Elsevier Ltd. All rights reserved.