The speciation reactions that take place in mixtures of water (H2O), carbon dioxide (CO2), and alkanolamines make the modelling of the chemical and fluid-phase equilibria of these systems challenging. We demonstrate for the first time that the statistical associating fluid theory (SAFT), formulated within a group-contribution (GC) framework based on transferable intermolecular square-well (SW) potentials (SAFT-γ SW), can be used to model successfully such complex reacting systems. The chemical reactions in these mixtures are described via a physical association model. The concept of second-order groups is introduced in the SAFT-γ SW approach in order to deal with the multifunctional nature of the alkanolamines. In developing the models, several compounds including ethylamine, propylamine, ethanol, propanol, 2-aminoethanol, and 3-amino-1-propanol are considered. We present calculations and predictions of the fluid-phase behaviour of these compounds and a number of their aqueous mixtures with and without CO2. The group-contribution nature of the models is used to predict the absorption of CO2 in aqueous solutions of 5-amino-1-pentanol and 6-amino-1-hexanol. The proposed predictive approach offers a robust platform for the identification of new solvents and mixtures that are viable candidates for CO2 absorption, thereby guiding experimental studies.