Ca(OH)(2) was activated towards SO2 by hydrating it with various fly ashes collected from three different coal-fired power plants in Turkey. Reactive Ca(OH)(2)-fly ash sorbents were prepared using both atmospheric and pressure hydration techniques. The hydrated sorbents were analyzed using mercury porosimeter, thermogravimetric analyzer and X-ray diffractometer. The relative surface area increments of sorbents are influenced by the hydration conditions and the source of the fly ash. The reactive species formed during hydration as detected with X-ray diffraction measurements, were found to be responsible for these surface area increments. Thermogravimetric measurements showed that decreasing the Ca(OH)(2) content of the hydrated sorbent which was presumably converted to Ca-containing reactive species caused an increase in the surface area of the sorbent. It was concluded that the reactivity of hydrated sorbents towards SO2 closely related to the sorbent surface area and the presence of the reactive species in the sorbent. (C) 1998 Elsevier Science B.V.