An equation for the adsorption of a mixture of electrolytes on a solid adsorbent has been derived in terms of average activity coefficients. The equation derived differs from the classical Gibbs equation by the presence of the second term that reflects the specific character of a solid, i.e. it takes into account the possible deformation of the adsorbent during adsorption and the difference between the mechanical and thermodynamic surface tension in a solid. The equation has been converted in a form in which it is applicable to the description of membrane and electrochemical equilibria near the surface of a solid.