The retention of SO2 by Ca(OH)(2)-derived CaO has been studied at 573 K. The thermal treatment of Ca(OH)(2) forms CaO sorbents with high activity to retain SO2. At the temperature of 573 K, it has been proved that SO2 interacts with CaO to form a surface CaSO3 specie. The regeneration process by thermal decomposition of CaSO3, giving SO2 and CaO, has been examined using techniques such as in situ FTIR and in situ XRD. In addition, CO2 chemisorption and SO2 sorption and sorbent regeneration was evaluated by thermogravirnetry. The thermal regeneration process used to decompose CaSO3 causes an important loss in the SO2 retention capacity of the sorbent. Using several experimental procedures, a series of reactions have been analysed (Ca(OH)(2) decomposition, formation of superficial CaSO3, change from surface to bulk CaSO3 and CaSO3 decomposition) and an insight into the regeneration process has been obtained. The decrease in SO2 retention, following the thermal regeneration step, is due to both an increase in the particle size and the conversion of CaSO3 into CaSO4 and CaS (CaSO3 disproportionation) upon heat treatment. The increase in the particle size is responsible for about 88% of the loss of activity, whereas CaSO3 disproportionation explains a 12% of loss.