The hydrolytic stability of C-18 monolayers supported on TiO2 and ZrO2 was studied. Three types of monolayers were prepared from the following octadecyl modifiers: (1) octadecyldimethylchlorosilane (C18H37Si(CH3)(2)Cl); (2) octadecylsilane (C18H37SiH3) and (3) octadecylphosphonic acid (C18H37P(O)(OH)(2)). The hydrolysis of the surfaces prepared was studied under static conditions at 25 and 65 degreesC at pH 1-10. On the basis of the loss of grafted material, the stability of the monolayers fall in the following range: C18H37P(O)(OH)(2) greater than or equal to C18H37SiH3 much greater than C18H37Si(CH3)(2)Cl. At 25 degreesC, monolayers from C18H37P(O)(OH)(2) showed only similar to2-5% loss in grafting density after one week at pH 1-10. The high stability of these monolayers was explained because of the strong interactions of the phosphonic acids with the substrates. Monolayers from C18H37Si(CH3)(2)Cl showed poor hydrolytic stability at any pH, which was explained because of the low stability of Ti-O-Si and Zr-O-Si bonds. Unlike monofunctional silanes, trifunctional silane (C18H37SiH3) yielded surfaces that showed good hydrolytic stability. This suggests that the stability of the monolayers from trifunctional silanes is primarily due to "horizontal" bonding (Si-O-Si or Si-OH...HO-Si) rather than due to bonding with the matrix (M-O-Si). At 65 degreesC, all C-18 surfaces become more susceptible to hydrolysis; however, the trend observed for 25 degreesC remained unchanged. Low-temperature nitrogen adsorption was used to study the adsorption properties of the monolayers as a function of their grafting density. The energy of adsorption interactions showed a significant increase as the grafting density of the monolayers decreased. The order of the alkyl groups in the monolayers, as assessed from CH2 stretching, decreased as the grafting density of the monolayers decreased.