The reactivity of silica gel and Stober silica toward molecular and anionic bases of various size and strength was examined in aqueous medium. The extent of the deprotonated silanol groups was discussed with respect to the size, strength, and concentration of the reactants, as well as to the nature of the silica. Measurements of the complex impedance performed on the decanted samples were found to be a rapid, sensitive, and nondestructive way for the in situ characterization of the surface silica ionization process in aqueous alkaline solutions. The apparent equilibrium constant for the deprotonation reaction was found to be strongly affected by the degree of dissociation of both silica samples, so that the complete neutralization was never observed. The molecular-sieving properties of the Stober silica was demonstrated experimentally by using bases of various dimensions: while the ionization of silica gel was nearly unaffected by the base size, that of the monodispersed silica beads was strongly influenced by the ability or inability of the base to enter the bulk. of the beads. The use of reactants larger than 0.3 nm totally prevents accessibility to the internal silanols. Reactivity of bases of critical dimensions ranging from 0.2 to 0.3 nm, though they were able to diffuse freely within the porous structure, was found to be hindered by volume exclusion effects.