Micron-sized CaO, obtained by calcination of mesoporous CaCO3, attained 36 wt % CO2 sorption capacity after 100 cycles of carbonation and calcination reactions at 700 °C. The extent of simultaneous carbonation (X CO 2 ) and sulfation (X SO 2 ) of CaO at 700 °C was obtained under simulated flue gas conditions (10% CO2, 3000 ppm of SO2, 4% O2 in N2). CaO reacts with SO2 to form thermally stable CaSO4, which leads to a reduction in the CO2 capture capacity of CaO. Whereas X SO 2 increases monotonically with the residence time, X CO 2 goes through a maximum and eventually drops as a result of direct sulfation of CaCO3. The maximum value attained by X CO 2 was 50% in 10 min in the first cycle. The highest X CO 2 /X SO 2 ratio of 5 is attained at a residence time of 5 min. X SO 2 is higher under simultaneous carbonation and sulfation conditions compared to sulfation of CaO or direct sulfation of CaCO3.