The influence of CO2 and O-2 in the flue gas on the reaction of hydrated lime sorbent with SO2, was studied using a fixed-bed differential reactor under conditions prevailing in the spray-drying flue gas desulfurization process. With the presence of CO2, the sulfation and carbonation reactions of Ca(OH)(2) took place simultaneously until Ca(OH)(2) ceased to react. The CaCO3 produced reacted further to form CaSO3 . 1/2H(2)O. The apparent sulfation rate, total reaction rate, and final total conversion of Ca(OH)(2) were greater than those for the case without CO2. The final total conversion was about 1.45 times that for the latter case at the conditions of 1000 ppm SO2, 60 degrees C, and 70% relative humidity. The same effect was observed whether CO2 was present with O-2 or not. Kinetic expressions obtained by assuming chemical reaction control and considering the surface coverage by product crystals best represented the kinetic data.