A theory accounting for the effect of the counterions on the adsorption constant, K, is proposed. The experimental values of K were determined by using surface and interfacial tension isotherms measured by us or available in the literature. By accounting for the adsorption energy u0 of the counterion, a generalization of Gouy equation and a modified expression for the adsorption constant, K, are derived. The adsorption energy is calculated from the London equation, which involves the polarizabilities α0i and the ionization potentials Ii of the respective components and the radius Rh of the hydrated ion. By careful analysis of the available experimental data for α0i, Ii and Rh, coupled with some reasonable hypothesis, we succeeded to obtain linear dependences between the calculated values of u0 and the experimental data for lnK with slopes rather close to the theoretical ones. The obtained results for u0 were used to calculate the disjoining pressure isotherms of foam films stabilized by DTAF, DTACl and DTABr. It turned out that the type of the counterion has significant effect on the disjoining pressure.