The electrolyte-nonrandom two-liquid model has been used to represent the thermodynamic behavior of the system : methyldiethanolamine-CO2-H2S-water. The Data Regression System (DRS) of Aspen Plus was used to regress parameters of the model to experimental data. pH and conductivity data were utilized to supplement vapor-liquid equilibria (VLE) data and improve confidence in model predictions at low acid gas loadings. Predictions for the mixed acid gas systems can be accurately made from the single acid gas parameter sets without the need to regress additional parameters. VLE data were fit well and the calculated heat of absorption matches calorimetric data.