A thermodynamic treatment of equilibria of protolytic reaction involving ligands immobilized at a rigid matrix surface is put forward. A shift observed in equilibrium constants of such reactions related to analogous reactions of free ligands is qualitatively explained. On the basis of the generalized mass action law, an attempt is made at accounting for the influence of the electric double layer disturbed by the spatial dispersion of the dielectric constant of the adjacent solution on the equilibrium state of surface reactions. In so doing, the electrostatic contribution to the surface tension of modified matrices is calculated using Derjaguin’s formula for the free electrostatic energy of a charged surface in contact with an electrolytic solution.