By combining density functional theory and lattice phonon dynamics, the thermodynamic properties of CO 2 absorption/desorption reactions with dehydrated potassium carbonates through K 2CO 3·1. 5H 2O + CO 2 = 2KHCO 3 + 0.5H 2O(g) are analyzed. The energy change and the chemical potential of this reaction have been calculated and used to evaluate its thermodynamic properties and phase transitions. The results indicate that the K 2CO 3·1. 5H 2O can only be applied for postcombustion CO 2 capture technology at temperatures lower than its phase transition temperature, which depends on the CO 2 pressure and the steam pressure with the best range being P H2 O ≥ 1.0 bar. Above the phase transition temperature, the sorbent will be regenerated into anhydrous K 2CO 3. If the steam pressure P H2 O is much greater than 1.0 bar, it is possible to use the K 2CO 3·1. 5H 2O sorbent for precombustion CO 2 capture technology. Compared to anhydrous K 2CO 3, K 2CO 3·1.5H 2O requires less energy for regeneration.