The interfacial tension between H2O and [(1 - x)n-decane + xCO(2)] was investigated for three different compositions of CO2 in the alkane-rich phase, of mole fractions x = (0.0, 0.2, and 0.5), along several isotherms at temperatures up to 443 K and pressures ranging from the miscibility state points for (n-decane + CO2) up to 50 MPa. The pendant drop method was implemented using a high pressure apparatus consisting of a view cell, fitted with a high pressure capillary tube for creating pendant H2O drops in the [n-decane + CO2] bulk phase. The conditions investigated cover a wide range relevant to carbon storage, providing information as to how the variation of the CO2 content in hydrocarbon fluids affects their interfacial tension with aqueous phases, which influences the trapping potential of underground formations. The results were compared to literature values where possible. A thermodynamic analysis of the dependence of the interfacial tension on CO2 composition, as well as on temperature, was discussed with respect to the Gibbs surface excess concentration and the enthalpy of interface formation, respectively. The present work provides novel interfacial tension data for the ternary system and addresses possible reasons for the observed discrepancies in literature values for the binary (H2O + n-decane) system observed over the range of conditions investigated. The reported results have a relative average standard deviation of 1.7 %.