The solubility of carbon dioxide in aqueous solutions of methyldiethanolamine (MDEA) sulfate (1 m), in aqueous solutions of MDEA and MDEA sulfate (2 m and 1 m, respectively), and in aqueous solutions of MDEA and sodium sulfate (2 m and 1 m, respectively) was measured at 313 K to 413 K and total pressures up to 10.6 MPa. The experimental results were used to extend and test a model for the thermodynamic equilibrium encountered in the solubility of carbon dioxide in such aqueous solutions.