In this work, a comprehensive model is developed for the absorption of carbon dioxide into aqueous mixtures of primary or secondary alkanolamines with tertiary alkanolamines. The model, which is based on penetration theory, incorporates an extensive set of important reversible reactions and takes into account the coupling between chemical equilibrium, mass transfer, and chemical kinetics. The reaction between CO2 and the primary or secondary amine is modeled according to the zwitterion mechanism. The key physicochemical properties that are needed for the model are the CO2 physical solubility, the CO2 and amine diffusion coefficients, and the reaction rate coefficients and equilibrium constants. Data for carbon dioxide absorption into aqueous mixtures of diethanolamine and methyldiethanolamine are compared to model predictions.