A fractional monolayer film of ions is formed by the adsorption of a gas onto a metal substrate. These ions form a discrete dipole layer, the potential across it being the change in electron work function of the surface. As additional ions are brought to the surface, they penetrate only a fractionfof the potential across the dipole layer.In this work the penetration coefficientfvalid for the entire range of coverages is derived for two limiting types of films, immobile and mobile. By considering thermal effects, a penetration coefficient for partially mobile films is also derived. The values obtained for the penetration coefficient are applicable to any substrate‐adsorbate system.The theoretical expressions for the penetration coefficient are evaluated for cesium on tungsten and compared with the data of Taylor and Langmuir.