Inconsistent trends for the physical solubility of carbon dioxide in aqueous methyldiethanolamine (MDEA) are presented. These inconsistencies are found between data sets for the chemical solubility of carbon dioxide in aqueous MDEA. In order to rationalize this inconsistency, data are presented for the solubility of nitrous oxide and carbon dioxide in MDEA solutions neutralized with sulfuric acid. The physical solubility is seen to decrease with increasing ionic strength. Previously published models show the incorrect trend for the physical solubility of carbon dioxide as a function of loading because of this discrepancy in the data. The electrolyte-NRTL model was successfully used to model the chemical and physical solubility of carbon dioxide in MDEA by defining the reference state for solutes as infinite dilution in the aqueous phase instead of the mixed solvent. VLE data at high temperature and high loading are needed for industrially important MDEA concentrations. The N2O analogy needs to be studied further in solutions at high and moderate loading as a function of temperature.