Ca(OH)(2)/fly ash sorbents were characterized and tested for reactivity toward SO2 at conditions similar to those in the bag filters in the dry and semidry flue gas desulfurization processes. The sorbents were mesoporous, and their specific surface areas linearly correlated with their specific total pore (1.7-300 nm) or mesopore (2-50 nm) volumes. The 1 h utilization of Ca in a sorbent increased, in general, with increasing fly ash content. Sorbents with Ca(OH)(2) contents larger than 33 wt % had higher weights of SO2 captured per unit weight of sorbent than Ca(OH)(2) had alone, and the sorbent with 70 wt % Ca(OH)(2) had the maximum capture. The 1 h utilizations of Ca for the sorbents and Ca(OH)(2) increased almost linearly with increasing specific surface area or pore volumes of the sorbent. The SO2 captured per unit surface area or pore volume of the sorbent decreased almost linearly with smaller Ca(OH)(2) content. The reactivities of the sorbents were enhanced mainly by their large specific surface areas and to a lesser degree by the reactive constituents in fly ash.