The hydrophilic surface of porous glass substrates became highly hydrophobic (static contact angle, theta(s)(water) = 119.4 +/- 0.7-degrees; theta(s)(n-hexadecane) = 80.7 +/-2.5-degrees) by chemical vapor surface modification (CVSM) with (heptadecafluorodecyl)trichlorosilane (HFTS). Diffuse reflectance Fourier-transformed infrared (FT-IR) spectra Of SiO2 powder used as a model adsorbent indicated that the HFTS molecules are anchored on the surface via condensation between their terminal functional groups and isolated surface Si-OH groups. The quite low reflectance of the pristine porous glass of 0.17 % at the wavelength of 490 nm was only increased to 0.62 % by means of the CVSM treatment, while the fluoroalkylsilane coating using a rubbing method raised it up to 4.5 %. The slight red shift (approximately 5 nm) of the wavelength having a minimum reflectance and data of FT-IR attenuated total reflection and X-ray photoelectron spectroscopies suggested that the HFTS molecules form a monolayer along the external and the internal surface, exposing mainly CF3 groups outermost in the CVSM sample. The Cassie-Baxter theory on the composite surface consisting of open area and hydrophobic region covered with HFTS was reasonably used to account for the excess increase of the contact angle over that in the smooth surface.