(Liquid + vapour) equilibrium and density measurements have been carried out on {xAr + (1 - x)C2H6}(l) at the temperature 90.69 K (the methane triple-point temperature). The excess molar Gibbs free energy G(m)E and the excess molar volume V(m)E have been calculated. A critical examination of the various sets of (liquid + vapour) equilibria available in the literature has been performed, and the internal and relative consistencies of the several sets of results assessed through a maximum-likelihood technique. The values of the excess molar thermodynamic functions have been interpreted in the light of some simple models.