Aqueous ammonia was investigated as a new absorbent of the chemical absorption process for CO2 capture from combustion flue gas. The effects of the temperature and concentration of aqueous ammonia on CO2 absorption in a semi-batch reactor were studied by interpreting breakthrough curves. Raman spectroscopy analysis of CO2 loaded aqueous ammonia provided concentration changes of bicarbonate, carbonate, and carbamate as well as CO2 sorption capacity at given time during the absorption with 13 wt% aqueous ammonia at 25 degrees C. It was observed that carbamate formation was dominating at the early stage of absorption. Then, the bicarbonate formation took over the domination at the later stage while the carbonate remained unchanged.