Thermogravimetric apparatus (TGA) and X-ray diffraction (XRD) have been used to study the characteristics of potassium-based sorbents for CO2 capture. The carbonation reactivity of K2CO3 center dot 1.5H(2)O and K2CO3 dehydrated from K2CO3 center dot 1.5H(2)O was weak. However, K2CO3 calcined from KHCO3 showed excellent carbonation capacity and no deactivation of sorbents during multiple cycles. The XRD results showed that the sample dehydrated from K2CO3 center dot 1.5H(2)O was K2CO3 with structure of monoclinic crystal (PC#1). The carbonation products of PC#1 included K2CO3 center dot 1.5H(2)O and KHCO3, and K2CO3 center dot 1.5H(2)O was the main product. Correspondingly, K2CO3 with structure of hexagonal crystal (PC#2) was the product calcined from KHCO3, and the main carbonation product of PC#2 was KHCO3, The byproduct of K4H2(CO3)(3)center dot 1.5H(2)O for PC#2 would affect the carbonation processes. Hydration tests confirmed the two hypotheses: the hydration reaction will first occur for K2CO3 with structure of monoclinic crystal, and the carbonation reaction will first occur for K2CO3 with structure of hexagonal crystal. The reaction principles were analyzed by product and the relevant reactions. This investigation can be used as basic data for dry potassium-based sorbents capturing CO2 from flue gas.