A thermodynamically consistent model recently developed for representing CO2 solubility in aqueous solutions of methyldiethanolamine is extended to include H2S and lower hydrocarbons. The electrolyte-nonrandom two-liquid (NRTL) activity coefficient model and the PC-SAFT equation of state are used to represent the liquid phase and the vapor phase nonideality, respectively. The Henry's constant of H2S in water is fitted to the total pressure data of the H2S-H2O binary system. The NRTL binary interaction parameters of the solvent-electrolyte binaries are regressed with the VLE data of the H2S-H2O-MDEA ternary system. The differential heat of H2S absorption of the H2S-H2O-MDEA ternary system and the partial pressures of H2S and CO2 of the H2S-CO2-H2O-MDEA quaternary system are predicted and compared favorably to the experimental data. The Henry's constants of light hydrocarbons in water and MDEA are also determined by fitting the total pressure data of the hydrocarbon-H2O binary and the hydrocarbon-H2O-MDEA ternary systems.