The disposal of acid gas (CO2/H2S mixtures) is a critical aspect in the production of hydrocarbons from sour gas fields. The increasing emphasis on CO2 sequestration has also renewed interest in the disposal of flue gas mixtures (primarily containing CO2 and H2S). A common strategy for safe disposal, in either case, is to inject the acid gas in aquifers close to production plants. These strategies rely on solubility calculations at different pressures and temperatures, governed by the field operating conditions. We present a comprehensive approach using Gibbs free energy minimization to calculate acid gas solubility in water at high temperatures (298-393 K) and pressures (0.1-80 MPa). The advantage of this approach is the flexibility to use different thermodynamic models for different phases. The proposed model uses the Peng-Robinson (PR) Equation of State (EOS) description for gas components while the liquid components are described using the ideal assumption for the temperature range 298-323 K and the Nonrandom Two-Liquid (NRTL) activity coefficient model at temperatures greater than 323 K. The model predictions compare well with experimental data for binary (CO2-H2O and H2S-H2O) and ternary mixtures (CO2-H2S-H2O). The model can also be easily extended to predict the solubility of any gas in water as well as brine containing ions to incorporate geochemical reactions.