Thermodynamic properties of the CO2–brine pseudo-binary system are essential for the design of geological carbon storage (GCS) projects, especially those utilizing saline aquifers. The gas–liquid–solid interactions manifest in the interfacial tensions (IFTs) and contact angle determine the injectability, sealing capacity, and storage security of the GCS process. Dissolution of CO2 in the reservoir brine occurs throughout the entire GCS process, leading to enhanced storage capacity but also to acidification of the brine, possibly leading to reservoir or seal damage. Two of the most important thermodynamic properties of the fluids are the mutual solubility and the IFT of the CO2–brine pseudo-binary system. In this work, we report a new correlative model for the IFT between CO2- and water-rich phases over wide ranges of temperature (273 to 473 K) and pressure (up to 100 MPa). The model is parameterized for brines comprising any combinations of sodium, potassium, calcium and magnesium cations with chloride, sulphate and bicarbonate anions up to a total molality of at least 5 mol·kg−1. The independent variables in this new model are reduced temperature, ion molalities and the mole fraction of CO2 dissolved in the aqueous phase. The latter is related to temperature, pressure and ion molalities by an improved model for the mutual solubility. More than 2000 experimental data points were used in the development of the two models. For the IFT of the CO2-H2O binary system, the overall root-mean-square deviation (RMSD) is 0.65 mN·m−1 while the absolute average relative deviation (AARD) is 1.8%. In the case of mutual solubility, the RMSD of CO2 mole fraction in the aqueous phase is 0.0003 and the AARD is 5.5% while, in the non-aqueous phase, the RMSD of H2O mole fraction is 0.0035 and the corresponding AARD is 8.7%. Similar results are found for the CO2-brine systems.