To choose the proper active component for potassium-based sorbents to capture CO2, the carbonation behavior of some potassium carbonate from different sources was investigated with thermogravimetric analysis (TGA) and a fixed-bed reactor. The results show that the carbonation reaction process and the reaction rate of the samples are different, although their crystal structures and elemental compositions are the same. K2CO3 calcined from KHCO3 shows excellent carbonation capacity through producing the active species of K4H2(CO3)(3)center dot 1.5H(2)O. The dimensionless weight increased to 1.36 in 27.7 min in TGA, corresponding to 80.2% conversion of the K2CO3 to KHCO3. The amount of CO2 capture reached 6.74 mol/(kg sorbent) within 23.3 min, corresponding to 93.0% conversion of the K2CO3 to KHCO3. The reason is that the particle morphologies of those sorbents are different: the surface area and pore volume of K2CO3 calcined from KHCO3 are the highest, and the pore distribution range is wider than that of other samples. Therefore, it is better to load potassium carbonate onto a support with a large surface area and pore volume, to improve the CO2 capture capacity.