The interfacial tensions (IFTs) between hydrocarbon-, water- and CO2-rich phases are important in the processes of carbonated water injection for enhanced oil recovery and carbon geological storage. In this work, the IFTs between decane-rich and water-rich phases, in the presence of the third CO2-rich phase, were measured by the pendant drop method and modelled with the density gradient theory at temperatures from 298.15 K to 353.15 K and at pressures up to the critical point pressure of (CO2 + decane) (maximum 11 MPa at 353 K). The dynamic IFT decreased over time due to CO2 adsorption on the interface and mass transfer into the decane-rich drop until equilibrium was reached. As expected, the equilibrium IFTs were observed to decrease with increasing pressure. At low pressures, the equilibrium IFTs decreased with increasing temperature while, at high pressures, the reverse was observed. By comparing the IFTs of ternary (decane + CO2 + H2O) system with those of binary (decane + H2O) system, it was found that CO2 could reduce system IFT, for example by 15.2 mN/m at T = 333 K and p = 10 MPa. The extent of reduction depends on the solubility of CO2 in the liquid phases and the extent of adsorption at the interface. Furthermore, the equilibrium IFT was found to be linearly dependant on the CO2 concentration in the water-rich phase isothermally, and an empirical relation was developed with an average absolute deviation of 0.3 mN/m. Density gradient theory coupled with the volume translated CPA equation of state was found to provide an accurate description of the IFTs with an average absolute deviation of 0.8 mN/m, proving its capability of predicting IFTs of (alkane + CO2 + water) in the three-phase region. The density profile in the interfacial region is also demonstrated. There is an enrichment of CO2 molecules at the interface and the enrichment is more pronounced with increasing pressure or decreasing temperature.