Dogas and Rochelle (2011) measured CO2 mass transfer rates in 7-13 molal aqueous monoethanolamine (MEA) and 2-12 molal piperazine (PZ) from 40 to 100 degrees C over a large range of CO2 loading. They observed that the liquid phase mass transfer coefficient (k'g) was almost independent of amine concentration and temperature. In this paper models are created to explain this behavior. CO2 reaction rates in MEA and PZ are represented with termolecular (base catalysis) kinetics with activity-based rate expressions. Solving the activity-based rate expressions with a shell balance and implementing diffusion resistance using film theory yielded an expression for the liquid phase mass transfer coefficient, kg. Parameters in the k'g expression were estimated from existing literature data. Two pre-exponential rate constants (k(MEA) and k(PZ)) are the only parameters that were adjusted to match experimental data. Estimates from independent sources of parameters in the model for kg fully account for the observed effects of CO2 loading, temperature, and amine concentration. The k'g expressions match the 93 wetted wall column experimental rates measured by Dogas and Rochelle (2011) with average deviations of 13% for MEA and 19% for PZ. The mass transfer expressions also match experimental data obtained by other researchers. This model shows that complex rate behavior in MEA and PZ systems can be fully explained using existing literature data. It also shows that the MEA and PZ systems can be represented by termolecular kinetics on an activity basis. (C) 2011 Elsevier Ltd. All rights reserved.