Experimental interfacial tensions of the systems (H2O + CO2), (H2O + N-2), (H2O +Ar), (H2O + CO2 + N-2) and (H2O + CO2+ Ar) are compared with calculations based on the statistical associating fluid theory for variable range potentials of the Mie form (SAFT-VR Mie) in combination with the square-gradient theory (SGT). Comparisons are made at temperatures from (298 to 473) K and at pressures up to 60 MPa. Experimental data for the systems (H2O + CO2), (H2O + N-2) and (H2O + CO2 + N-2) are taken from the literature. For the (H2O +Ar) and (H2O + CO2 +Ar) systems, we report new experimental interfacial-tension data at temperatures of (298.15-473.15) K and pressures from (2 to 50) MPa, measured by the pendant-drop method. The expanded uncertainties at 95% confidence are 0.05 K for temperature, 70 kPa for pressure, 0.016 x gamma for interfacial tension in the binary (Ar + H2O) system and 0.018 x gamma for interfacial tension in the ternary (CO2 + Ar + H2O) system. The parameters in the SAFT-VR Mie equation of state are estimated entirely from phase-equilibrium data for the pure components and binary mixtures. For pure water, the SGT influence parameter is determined from vapour-liquid surface-tension data, as is common practice. Since the other components are supercritical over most or the entire temperature range under consideration, their pure-component influence parameters are regressed by comparison with the binary interfacial-tension data. A geometric-mean combining rule is used for the unlike influence parameter in mixtures without incorporation of adjustable binary parameters. The SAFT-VR Mie + SGT approach is found to provide an excellent correlation of the surface tension of water and of the interfacial tensions of the binary systems comprising H2O with CO2 or Ar or N-2. When applied to predict the interfacial tensions of the two ternary systems, generally good results are found for (H2O + CO2 + N-2) while, for (H2O + CO2 + Ar), the theory performs well at hightemperatures but significant deviations are found at low temperatures. Overall, the SAFT-VR Mie + SGT approach can be recommended as a means of modelling the interfacial properties of systems comprising water, carbon dioxide and diluent gases. (C) 2015 Elsevier B.V. All rights reserved.