The solubilities of CO2 and H2S mixtures in methyldiethanolamine aqueous solutions were investigated in the wide temperature range from 313.15 (absorption range) to 403.15 K (stripping range) using the static method. The values of equilibrium constants for amine protonation (K-1), activity coefficients (gamma(i)), and fugacity coefficients (Phi(i)) were regressed from the newly obtained experimental solubility data to correct nonideality. In order to obtain the equilibrium constants for amine protonation, modified Kent-Eisenberg model was applied. Then, two kinds of activity coefficient models, the Deshmukh-Mather model and the electrolyte nonrandom two-liquid model, were applied to minimize the gaps between experimental data and predicted values with newly obtained equilibrium constants. In this study, six thermodynamic models were presented by combining (1) equilibrium model and (2) activity coefficient models. Based on the best fitted solubility results, the mole fractions of all electrolytes, the pH in the liquid phase, and the heat of absorption for CO2 and H2S were evaluated.