Recently, reliable pair-potentials have been obtained for argon, krypton and xenon. Pair-potentials of similar form are obtained for the argon-krypton and krypton-xenon interactions. Three parameters are adjusted to give experimental values for the long-range r-6 term and the excess free energy and excess volume for an equimolar liquid mixture at zero pressure and a specified temperature. The excess thermodynamic properties are calculated from the Barker-Henderson perturbation theory, which has been shown to give good values for the excess thermodynamic properties of mixtures of 6: 12 molecules, with the contributions of three-body forces and quantum effects included. Excellent agreement with the excess properties of argon-krypton and krypton-xenon mixtures is found at all concentrations. © 1975 Taylor & Francis Group, LLC.