By introducing a temperature dependent interchange parameter into a semi-empirical model for the Gibbs energy of mixing of liquid Na–Cs it is shown that the total entropy of mixing can be resolved into a hard sphere and an interaction energy contribution, both of which can be calculated from experimental data. By calculating the contribution to the entropy from the variation of molar volume with concentration it is therefore possible to obtain experimental values of (ΔSη +Sσ), the contribution from the packing and misfit of the hard spheres. This sum can be fitted by caesium and sodium hard sphere diameters which change by less than 1.0% over the whole concentration range.