The effect of temperature and O-2 concentration on the product distribution of the CaO-SO2 reaction was thermogravimetrically studied. The accompanied changes in pore characteristics (specific surface area and pore volume) with reaction temperature, time and O-2 concentration were also investigated. Among the sulfation products, CaSO3 was found to be thermally stable at temperatures below about 973 and 1015 K with and without O-2, respectively. Within CaSO3 stability limits, O-2 presence increased the initial sulfation rate, but virtually suppressed SO2-sorption capacity. On the other hand, at temperatures beyond CaSO3 stability limits, O-2 presence improved the initial sulfation rate as well as SO2-sorption capacity, and the reaction order with respect to SO2 was determined to be around unity independent of O-2 presence. Regardless of O-2 presence, the reaction order with respect to SO2 was found to increase from 0.5 to 1.1 with temperature increase from 823 to 1123 K. No influence of O-2 concentration on the initial reaction rates and sorption capacities were observed above 2 vol% O-2 in the tested temperature range (823-1123 K). The temperature dependency of the CaO-SO2 reaction system was correlated with an activation energy, E, ranging between 31 and 51 kJ.mol(-1).