Alkali carbonates supported on activated carbon (AC) for CO2 sorption from moist flue gases by a temperature-swing operation have been proposed. The temperature dependence of CO2 uptake by the sorbent was investigated to understand the CO2 sorption/release behavior. The CO2 sorption behaviors of alkali carbonates-on-AC at various temperatures were well explained based on CO2 gas-sorbent equilibria. However, the reverse reaction of thermal decomposition of alkali hydrogencarbonates, which were the reaction products after CO2 sorption, did not explain the sorption behaviors. The CO2 gas-sorbent equilibrium constants were used to determine the effect of CO2 partial pressure in the feed gas on the CO2 sorption capacities. The results showed that the pressure-swing operation was appropriate for sorption using alkali carbonates-on-AC. At low CO2 partial pressures in the feed gas, the CO2 sorption capacities achieved using alkali carbonates-on-AC were higher than that achieved using K2CO3 solution (i.e., the Benfield process).