In this work, the solubility behavior of salts NaCl and NH4Cl was investigated in aqueous diethanolamine (DEA) solution by varying both temperature and the solvent composition. DEA has been prevalently used in the postcombustion process to capture CO2, but a limited amount of references can be found pertaining to any solid liquid system containing DEA. It was experimentally determined that the solubility of NaCl decreases, whereas the solubility of NH4Cl increases with the addition of DEA (x'(DEA) = 0-0.2). The solubility of both salts increases if temperature is raised from 283 to 353 K. The common-ion effect was examined for the quaternary system when both salts are present in the solution. By the regression of solubility data with the Mixed Solvent Electrolyte model, we proved that readjusting the middle-range interaction parameters between Cl--DEA and DEAH(+)-Cl- would lower the relative deviation (within 5%) for each proposed ternary system. A comprehensive chemical model was further constructed based on the newly adjusted interaction parameters, provided to further analyze system equilibria, as well as the activity coefficients for aqueous electrolytes.