Smooth and porous glass surface substrates were modified with fluoroalkyltrichlorosilanes (CF3(CF2)(n)-CH2CH2SiCl3, n = 0, 3, 7, FAS) by means of chemical adsorption from the gas phase to give rise to highly hydrophobic surfaces. The hydrophobicity was found to increase with increasing molecular size of the adsorbates, accompanied by the more regular molecular orientation confirmed by the analysis of the X-ray photoelectron spectra (XPS). The resistance of the FAS layer to liquid H2O was examined by following the variations in the contact and critical tilting angles with time after immersion in boiling H2O. In all the samples tested, the contact angle (theta(s)) decreases and the critical tilting angle (alpha) increases with time. For the same FAS treatment, the decreasing rate of theta(s) is much smaller in the surface porous substrate, while the increasing rate of alpha is greater. These noticeable facts were rationalized in terms of the data of Fourier-transformed infrared attenuated total reflection and XPS measurements. Also, the former data reveal that the FAS molecules are anchored via interfacial Si-s-O-Si bonds and intermolecular Si-c-O-Si-c bonds in the monolayer formation process and partially desorbed by hydrolysis during immersion in H2O. Molecular orbital calculations of the clusters employed as a model of the FAS molecule chemically bonding to the surface of SiO2 were also carried out in order to obtain information on the electronic distribution and the reactivity with H2O, suggesting that the crucial step of the deterioration is the nucleophilic attack of the interfacial Si atoms accompanied by cleavage of the intermolecular Si-c-O-Si-c bonds. On the basis of these results, the deterioration mechanism was discussed.