Aqueous ammonia has been proposed as an absorbent for use in CO2 post combustion capture applications. It has a number of advantages over MEA such as high absorption capacity, low energy requirements for CO2 regeneration and resistance to oxidative and thermal degradation. However, due to its small molecular weight and large vapour pressure absorption must be carried at low temperature to minimise ammonia loss. In this work the rate of CO2 absorption into a falling thin film has been measured using a wetted-wall column for aqueous ammonia between 0.6 and 6 mol L-1, 278-293 K and 0-0.8 liquid CO2 loading. The results were compared to 5 mol L-1 MEA at 303 and 313 K. It was found that the overall mass transfer coefficient for aqueous ammonia was at least 1.5-2 times smaller than MEA at the measured temperatures. From determination of the second-order reaction rate constant k(2) (915 L mol(-1) s(-1) at 283 K) and activation energy E-a (61 kJ mol(-1)) it was shown that the difference in mass transfer rate is likely due to both the reduced temperature and differences in reactivity between ammonia and MEA with CO2. (c) 2009 Elsevier Ltd. All rights reserved.