A thermodynamic model to predict phase equilibrium of methane and carbon dioxide hydrate in aqueous electrolyte solutions is presented. Using the Pitzer model to account for the variation of water activity due to electrolytes and dissolved gas in aqueous solutions, we extended the model based on ab initio molecular potential developed recently by us for the CH₄-H₂O and CO₂-H₂O binary systems to the CH₄ (or CO₂)-H₂O-salts system. Comparison of the model with extensive experimental data indicates that this model can accurately predict the phase equilibrium of CH₄ hydrate and CO₂ hydrate in various electrolyte solutions (such as aqueous NaCl, KCl, CaCl₂, NaCl + KCl, NaCl + CaCl₂ solutions, and seawater) from zero to high ionic strength (about 6 m) and from low to high pressures.