A generalized expression for the potential drop across an adsorbed film developed in earlier work is used to interpret the adsorption potential shift at the free water surface and at the uncharged mercury/solution interface when they are composed of adsorbate and solvent molecules. It is shown that according to the theory the inverse 1/Delta(chi) of the adsorption potential shift should vary linearly with the inverse 1/theta of the surface coverage by the adsorbate. This prediction is verified in all the experimental systems analysed. The ratio rho = epsilon(A)/epsilon(S), where epsilon(A) and epsilon(S) are the "distortional" dielectric constants of the adsorbed layer when theta = 1 and theta = 0 respectively, is calculated from the intercept on the ordinate and the slope of the linear 1/Delta X vs. 1/theta plot. It is found that p < 1 at the uncharged Hg/solution interface and p greater than or equal to 1 at the free water surface. In order to explain this difference various models of the adsorbed layer are analysed and discussed.