Although the CO2 absorption rate in ammonia solution is essential for the design of a practical column for the capturing of CO2, debate continues regarding the values as well as proper mechanisms to interpret measured data. To resolve it, more data for CO2 absorption rate in ammonia solution are required. In addition, as most of the data focuses on the absorption rate in a CO2-free absorbent, data in partially loaded ammonia solutions are particularly required. Therefore, the CO2 absorption rates in aqueous ammonia solution with 0 and 0.1 CO2 loading were investigated with a wetted wall column in this study. The apparent CO2 absorption rate in a partially CO2 loaded ammonia solution was about 30% lower than that in an unloaded solution. After determining the apparent reaction rates and applying them to termolecular mechanism. The termolecular mechanism was revealed to be a reliable model in both unloaded and partially loaded ammonia solutions. To calculate the reaction rate constants, essential base components were also determined. H2O was negligible in the unloaded ammonia solution; whereas the OH- was negligible in the partially carbonated ammonia solution. This was due to the fact that the concentration of hydroxyl ions drastically decreased as the CO2 loading is increased. (C) 2014 Elsevier B.V. All rights reserved.