A thermodynamic model developed for CO 2 and H 2S solubilities in aqueous MDEA solution is extended to cover CO 2 and H 2S solubilities in aqueous DIPA solution, aqueous sulfolane–DIPA solution, and aqueous sulfolane–MDEA solution. The model makes use of the 2009 version of the electrolyte NRTL model for liquid phase activity coefficient calculations and the PC-SAFT equation of state for vapor phase fugacity coefficient calculations. The NRTL binary parameters for the molecule–electrolyte pairs required for the H 2O–DIPA–CO 2 ternary and the H 2O–sulfolane–DIPA–CO 2 quaternary are regressed against the solubility data of CO 2 in aqueous DIPA solution and aqueous sulfolane–DIPA solution, respectively. The NRTL binary parameters for the molecule–electrolyte pairs required for the H 2O–DIPA–H 2S ternary and the H 2O–sulfolane–DIPA–H 2S quaternary are regressed against the solubility data of H 2S in aqueous DIPA solution and aqueous sulfolane–DIPA solution simultaneously. The NRTL binary parameters for the electrolyte–electrolyte pairs involved in the H 2O–DIPA–CO 2–H 2S quaternary are regressed against the solubility data of the acid gas mixtures in aqueous DIPA solution. Likewise, the NRTL binary parameters for the sulfolane–electrolyte pairs required for the H 2O–sulfolane–MDEA–CO 2 quaternary and the H 2O–sulfolane–MDEA–H 2S quaternary are regressed against the solubility data of the acid gases in aqueous sulfolane–MDEA solution. The predicted enthalpies of acid gas absorption are compared favorably with the literature data available for the H 2O–DIPA–CO 2 system, the H 2O–DIPA–H 2S system, and the H 2O–sulfolane–MDEA–CO 2 system.