The horizontal polymerization of trichlorosilanes on silica is discussed with regard to its use in chromatographic separations to reduce silanol activity and improve hydrolytic stability. It is shown that the use of C1spacers, which were previously demonstrated to provide very low silanol activity, are resistant to a pH 10 solution for at least 36 hr. It is also shown that the adsorptivity of water by silica gel is an important factor in achieving dense horizontal polymerization.