Chemically modified Ca(OH)2 sorbents for SO2 control in utility boilers were tested in an electrically heated, bench-scale isothermal flow reactor, operated at between 700 and 1000 C and residence times of from 0.6 to 2 sec calculated from bulk gas flowrates. Novel surfactant-modified Ca(OH)2 (SM-Ca(OH)2) sorbents were compared to conventional Ca(OH)2 produced by dry hydration (DH-Ca(OH)2). Sorbents were activated in the flow reactor. The gas composition was 5 vol % oxygen with the balance nitrogen. Activated sorbents, SM-CaO and DH-CaO, were size classified with an inertial cascade impactor downstream of the flow reactor. The structure of each separated fraction (six trays plus preimpactor, D50 from 0.74 to > 11.9 micrometers) was characterized by nitrogen adsorption. For each size fraction measured, the surface area was higher for SM-CaO than for DH-CaO. The effect of thermal sintering was the increase of median pore size as a result of eliminating fine pores (below 100 A). Changes in the pore structure of Ca(OH)2 sorbents reacting with SO2 were also investigated. The effect of thermal sintering on pore structure of sorbents reacting with SO2 was eliminated. The degree of conversion was controlled by varying gas-phase mass transfer resistance (SO2 concentrations from 50 to 3000 ppm).