The performance of limestone-derived CaO during many (>1000 in some cases) calcination and carbonation cycles is reported. After 150 cycles, the calcium utilization during carbonation reached a minimum value between 4 and 17%, with the asymptotic level depending strongly on the carbonation time. With the aid of mechanistic studies including investigations on sorbent surface topology by SEM and mercury intrusion, a mechanism for pore evolution during the cyclic capture is proposed consistent with the experimental observations. (C) 2008 American Institute of Chemical Engineers.