The aim of this work is to identify the characteristics of natural carbonates which upon calcination generate an optimum material for use as a CO2-capturing sorbent in large-scale industrial CO2-producing sources. Nine different naturally occurring Ca/Mg carbonates were selected for this study. The carbonates were fully characterized by a variety of analytical techniques including atomic absorption and redox volumetry, for the chemical characterization of the carbonates, and optical and scanning electron microscopy (SEM), X-ray diffraction, and Fourier transform infrared spectroscopy, to determine their crystallinity, morphology, and the presence of impurities. They were then subjected to successive (up to 100) calcination/recarbonation cycles, and their conversion decay curves were interpreted on the basis of the physical and chemical characteristics of the parent carbonates. The textural development of the sorbents during cycling was studied by Hg porosimetry and SEM. Hardness tests were also conducted on selected samples. It was concluded that both carbonate purity and crystallinity are important parameters in determining the performance of the sorbents. The activity of all the sorbents tested turned out to be highly dependent on the pore structure of the calcines and their variation during cycling. In turn, the natural tendency of the sorbents to develop low surface areas (poor efficiencies) during cycling seems to be enhanced by the presence of moderate amounts of Mg.