As part of the global efforts for renewable energy development, carbon dioxide solubility measurements were provided for the geothermal industry. Solubility is obtained at high pressures (6-40 MPa), in aqueous phases reproducing the exploited geothermal resources, through sodium chloride (NaCl) and calcium chloride (CaCl2) synthetic brines (1.2 mol NaCl.kg H2O-1 and 0.2 mol CaCl2.kg H2O-1), at 333.15 and 453.15 K. Both mixed-salt and single-salt brines were analyzed to produce original results. A new stirred reactor, with variable volume, was conceived to maintain gas-liquid equilibria, while solubility measurements were performed by titration, with a relative uncertainty of 0.028. The experimental methods were validated with solubility analyses in pure water, at 333.15 K, and in sodium chloride and calcium chloride single-salt brines (1 mol.kg H2O-1), at 323.15 K. Then, 20 original data were obtained and compared to PHREEQC (pitzer.dat) calculation results. The measurements highlight that carbon dioxide solubility decreases when the aqueous phase salinity increases. This salting-out effect increases to about 23%, when observed in the analyzed mixed-salt brines at 333.15 K, and it is mostly caused by sodium chloride in these fluids. However, one can notice that calcium chloride in these brines promotes part of the phenomenon too.