Coal combustion in power plant produces fly ash. Fly ash may be used in water treatment to remove mercury (Hg2+) from water or to immobilize mercury mobile forms in silts and soils. Experiments were carried out on two kinds of fly ashes produced by two circulating fluidized bed plants with different chemical composition: silico-alummous fly ashes and sulfo-calcic fly ashes. For the two kinds of fly ashes, adsorption equilibrium were reached in 3 days. Furthermore, removal of mercury was increased with increasing pH. Sulfo-calcic fly ashes allow us to remove mercury more efficiently and more steady. The chemical analysis of fly ash surface was carried out by electron spectroscopy. The results show that mercury is bound to ash surface thanks to several chemical reactions between mercury and various oxides (silicon, aluminium and calcium silicate) of the surface of the ashes. (C) 2002 Elsevier Science Ltd. All rights reserved.