A theoretical description is presented for the equilibrium capillary height (h(eq)) of a solution between parallel, flat, solid surfaces which contain ionizable groups. The h(eq) is related to the change in gravitational potential energy, the intrinsic wettability of the un-ionized surface, and the free energy of formation of the ionizable surface in aqueous solution. The theoretical approach takes into account both the electrostatic free energy of charging the surface and the change in the free energy associated with the acid-base reactions of the surface sites. It is shown that the dependence of h(eq) on pH depends on the number of ionizable surface sites per unit area, the intrinsic acid-base dissociation constant (K-a(i)) ofthe surface sites, and the background electrolyte. The negative free energy change which accompanies the acid-base reactions dominates over the positive electrostatic free energy of charging the surface. Consequently, the overall free energy of ionization is negative, and an ionized surface is more wettable than an un-ionized surface. The theoretical description is applied to experimental values of h(eq) as a function of pH, measured between two heptylamine plasma polymer surfaces in the presence of I mM NaCl. The theoretical fit to the data indicates that the plasma polymer surface contains ca. 10(17) amines/m(2) and the pK(a)(i) of the amine groups is ca. 5. The surface site density is in reasonable accord with values obtained by both derivatization techniques and contact angle measurements. The pK(a)(i) is consistent with a low effective dielectric constant for the polymer-water interface.