Reductive decomposition of spent CaSO4 was studied using a packed-bed reactor to regenerate an alternative CaO sorbent. The reactor was operated at various process conditions including an increasing CO concentration, CO/CO2 concentration ratio (0.067-1), and temperature (1123-1273 K). In all cases, N-2 was used as a balancing gas. The regeneration of CaO from CaSO4 was found to be most effective in the CO-CO2-N-2 atmosphere and strongly depended on the CO/CO2 concentration ratio. At 1273 K, an apparent conversion value of 0.91 for the decomposition of CaSO4 to CaO was obtained in a 2 vol % CO and 30 vol % CO2 atmosphere. On the other hand, in a CO-N-2 atmosphere, CaS was predominantly produced. The SO2 absorption capacity of CaO regenerated from CaSO4 was higher than that of limestone-calcined CaO. A larger pore diameter of the regenerated CaO was considered to be responsible for the higher SO2 absorptivity.