It is shown that the isotopic separation factor,α, for a monatomic liquid in equilibrium with its vapour at low temperatures can be used to calculate the mean-square fluctuations of the intermolecular virial function. The only knowledge of the intermolecular energy that is required is the separation,σ, at which the pair potential is zero. If the additional assumption is made that this potential is of the Lennard-Jones (n, m) type then it is possible to relateαton, m, σ and the thermodynamic properties of the fluid phases. Hence recent experimental values of α for argon can be used to test potentials of the Lennard-Jones type without any assumption about the structure of the liquid.