The solubility of carbon dioxide and nitrous oxide in water + methyldiethanolamine and ethanol + methyldiethanolamine (MDEA) solutions was investigated over the whole range of amine concentrations. Experiments were performed in a reaction calorimeter at 293 K at constant atmospheric pressure. From the measurements of the solubilities of N2O in aqueous MDEA and of CO2 in pure amine, it was found that a "N2O analogy" for the prediction of CO2 solubility could not be used over the whole composition range of the aqueous MDEA solutions. In the case of ethanol + MDEA solutions, both gases show opposite trends for physical solubility as a function of amine concentration. These results disqualify the use of the "N2O analogy" in the estimation of CO2 solubility in ethanol + MDEA solutions.