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Abstract: The oil and gas industry produces large quantities of sour gases. In the perspective of the geological storage of acid gases in deep aquifers, the solubility of CO2 +H2S mixtures in pure water and in a 2M NaCl solution was studied under conditions typical of geological storage. Phase equilibrium measurements were undertaken at a temperature of 120°C and pressures ranging from 1.7 to 35MPa. Homogeneous mixtures of CO2 +H2S with H2S volume fractions of (0, 1/4, 1/3, 1/2, 2/3 and 1) were placed in contact with solution, and solubility was measured by continuous sampling of the solution. The protocol used was validated by comparison of the results obtained on pure gases with literature data. The mutual interaction between the gases in the mixture has a substantial influence on their respective solubilities. Equation-of-state models reproduce the decrease of CO2 solubility in the presence of H2S, but underestimate the exclusion of H2S by CO2. [Copyright &y& Elsevier]