[en] A simple model is proposed which describes the discontinuous phase transition and valence instability in SmS. It differs from other models in that, the instability due to increasing applied pressure, is completely described in terms of the semiconducting phase without using characteristics of the metallic phase. The electron-phonon interaction which renormalizes the pure electronic f to d transition hybridization generates the occurence of such behavior. It is shown that above a ratio between electron-phonon coupling and pure electronic f to d transition mixing, the semiconductor-metal transition is displayed. (author)