Literature values regarding the pressure dependence of the interfacial tension of the system of carbon dioxide (CO2) + water (H2O) show an unexplained divergence and scatter at the transition between low-pressure gas–liquid equilibrium and the high-pressure liquid–liquid equilibrium. We employ the Statistical Associating Fluid Theory (SAFT) and canonical molecular dynamics simulations based on the corresponding coarse grained force field to map out the phase diagram of the mixture and the interfacial tension for this system. We showcase how at ambient temperatures a triple point (gas–liquid–liquid) is expected and detail the implications that the appearance of the third phase has on the interfacial tensions of the system.