A simple two-state statistical mechanical model ascribing one configurational state each to the solvent and the adsorbate and site parity between them is analysed and solved under mean field approximation. The input parameters of the model are: (i) the permanent dipole moments of the solvent and the adsorbate; (ii) the respective polarizabilities; and (iii) the nearest neighbour distance. The interactions taken into account are: (a) the coulombic interactions due to permanent and induced dipole effects: (b) the discreteness of the charge effects; and (c) the specific substrate interactions. The resulting adsorption isotherm is non-congruent and more general than the classical Frumkin isotherm thereby providing a molecular basis for the latter. It is shown how a pseudo-Frumkin interaction constant that quadratically depends on the electrical variable can arise purely from polarizability differences between the solvent and the adsorbate. Many equivalent forms of the adsorption isotherm are also reported and the criteria for charge congruence and the potential congruence are analysed in the light of this new isotherm. © 1984 Elsevier Sequoia S.A.