CO 2 chemisorption, at different temperatures, has been studied in CaO and in the CaO-carbon system by means of simultaneous TG-DTA, MS, and XRD techniques. As a whole the different techniques used show that CO 2 chemisorbs in an irreversible manner and is mainly restricted to the surface of the CaO particles, provided that the chemisorption temperature is lower than 573 K. CO 2 chemisorption at 573 K can be used as a tool to measure the external surface area of CaO particles. Selective CO 2 chemisorption at 573 K has been applied to the CaO-carbon system in the field of catalytic carbon gasification. Several examples are given to show the usefulness of the catalyst surface area determined by CO 2 chemisorption in interpreting the catalytic activity of calcium in carbon gasification.