Although the reversible absorption of CO2 by CaO at high temperature has been proposed as a promising method for capturing and removing CO2 from hot gas streams produced by the combustion of fossil fuels and other industrial processes, the activity of the sorbent has invariably declined when applied over many cycles of CO2 absorption and desorption. The objective of the present investigation was to extend the life cycle performance of the absorbent through improved thermal pretreatment as the absorbent was prepared from limestone, dolomite, calcium acetate, or plaster of Paris. The first three materials were converted to CaO by calcination, while the third being an impure form of CaSO4 required a combination of reduction and oxidation. The life cycle performance of an absorbent derived by calcination was found to depend on the calcination conditions including particle size, temperature, atmosphere, and length of treatment. Sorbent stability was favored by calcining fine-size particles (10 mu m) at high temperature (1000 degrees C) in an atmosphere containing 50% CO2 or more. The sorbent derived from plaster of Paris by a cyclic process of reduction and oxidation at 1070 degrees C proved to be a very unusual material when applied because its activity increased rapidly and was still increasing after 200 cycles of absorption and regeneration.