Phase equilibria and thermodynamic properties of the quaternary H 2 S-CO 2 -H 2 O-NaCl system were studied using a statistical associating fluid theory (SAFT)-based equation of state (EOS) at temperatures from 0 to 200°C (373.15-473.15 K), pressures up to 600 bar (60 MPa) and concentrations of NaCl up to 6 mol/kgH 2 O. The understanding of the physical-chemical properties of this system is critical for predicting the consequences of co-injection of CO 2 and H 2 S into geological formations (geological carbon sequestration) as an option for mitigating the global warming trend. Equation of state parameters were generated from regression of available and reliable experimental data and incorporation of existing parameters for some subsystems. Densities were predicted and compared with available experimental results. Using the EOS developed in this study, we predicted equilibrium compositions in both liquid and vapor phases, fugacity coefficients of components, the equilibrium pressures at a given composition of the H 2 O-rich phase in electrolyte solutions with NaCl varying from 0 to 4 mol/ kgH 2 O, and the aqueous solution densities. These predicted values are tabulated and available as supplementary data in the electronic version online. These predictions provide information and guidance for future experiments regarding the thermodynamic properties and phase behaviors in the H 2 S-CO 2 -H 2 O-NaCl system.