In this work,hydrate phase equilibria were studied for 1%SO2+99%CO2 and 10%SO2+90%CO2 in three- (aqueous liquid-hydrate-vapor) and four-phase (aqueous liquid-hydrate-vapor-guest liquid) regions.Due to the Duhem’s theorem,incipient hydrate-forming temperatures were measured at constant given pressures and accurately predetermined overall loading compositions of all components.When compared with the CO2 hydrate phase equilibria on the pT plane,the addition of SO2 to CO2 shifted the incipient hydrate-forming conditions to higher temperatures at the same pressures.Also effects of the water contents were determined at the same ratio of 1%SO2+99%CO2 and 10%SO2+90%CO2,respectively.The phase equilibria were considerably affected by the water contents at 10%SO2+90%CO2 system.Increasing water contents decreased the incipient hydrate-forming temperatures resulting from the difference of their solubilities.Four-phase equilibria were measured and showed the pseudo-retrograde hydrate behavior.For thermodynamic modeling in the system of SO2+CO2+water,long range contribution should be considered since SO2 in aqueous solution is partially dissociated as weak electrolyte.In this work,the electrolyte equation of state based on hydrogen-bonding nonrandom lattice fluid theory (Electrolyte NLFHB EOS) [1] was employed and extended to the weak electrolyte solution.The model was found to successfully describe the phase equilibria of CO2+SO2+water mixtures with and without the hydrate phase.