Two consequences of Gibbs-Bogoliubov variational theory are examined as applied to liquid argon: the choice of a hard-sphere reference system leads to an equation of state if the packing fraction is used as the variational parameter, the non-ideal part of the entropy is only a function of eta . Calculations are performed along the freezing and vapour-pressure lines. Excellent agreement with experimental results is obtained if the Lennard-Jones potential is regarded as an effective pair potential and its parameters are appropriately chosen, and for the hard-sphere reference system the empirical radial distribution function proposed by Verlet and Weis (1972) is used together with the Helmholtz free energy derived from the Carnahan and Starling equation of state (1969).