A simple model for the phase transition in polystyrene latexes has been developed. The model assumes a priori that the transition is fundamentally caused by repulsion between the spherical latex particles, i.e., it is fundamentally hard sphere in nature, but that the hard sphere diameter of the polystyrene particles is in fact ionic-strength dependent. The resulting volume fraction vs. salt concentration phase diagram is in qualitative accord with the experimental phase diagram of Hachisu and co-workers.