A simple model for the free energy of mixing of a binary fluid is proposed using hard spheres as a reference system and a tail interaction between the unlike atoms. The model, firstly, provides an interpretation and rationale for the interchange energy parameterwand its temperature dependence occurring in the Flory formula and for a number of other current ideas. Secondly, it leads to a formula which appears to be a suitable next step in the refinement of the Flory model.