The absorption of carbon dioxide into aqueous ammonia solution using a packed column was investigated. The volumetric overall mass transfer coefficients (K(G)a(v)) for CO2 absorption into aqueous ammonia solutions were evaluated over ranges of main operating variables, that is, up to 15 kPa partial pressure of CO2, 61-214 m(3)/(m(2).h) gas flow rate, 0.76-3.06 m(3)/(m(2).h) liquid flow rate, 2%similar to 16% ammonia mass fraction, and 20-50 degrees C temperature. An empirical K(G)a(v) correlation for this system was proposed. Experimental results show that the mass transfer process in CO2 absorption into aqueous ammonia solution is mainly controlled by the resistance in the liquid phase and the ammonia concentration has a great effect on the overall mass transfer coefficient.