For the purpose of developing a CO2 separation process for flue gas of low CO2 partial pressure, a vacuum regeneration method for an absorbent with low CO2-loading was experimentally investigated. Since additional energy is consumed for water vaporization accompanied by CO2 desorption in the regeneration step, reducing the generation of water vapor becomes important for energy saving. A solution of potassium or sodium carbonate and bicarbonate mixture was used as a model solution of CO2-loaded absorbent; a carbonate solution containing diethanolamine (DEA) was also examined. Rates of CO2 desorption and water vaporization were obtained under the operating conditions of various pressures and temperatures. The rate ratio of water vaporization to CO2 desorption became minimum at a pressure equal to the vapor pressure of water. The rate of CO2 desorption at this pressure increased with temperature until 343 K and the increase was minimal beyond this temperature. The desorption rate was enhanced by the addition of DEA to the carbonate solution, and the rate reached a maximum at a high DEA concentration.