The reactions of boron trichloride, aluminum trichloride, and aluminum trimethyl vapors with the surfaces of characterized Aerosil silicas have been studied using infrared, mass spectrometric, and gravimetric techniques. Boron trichloride reacts completely with both the single and hydrogen-bonded surface hydroxyls. Exposure of the reacted solid to water vapor results in the reformation of the single SiOH groups and the production of surface B-OH groups. Aluminum trichloride vapor reacts with both the single and hydrogen-bonded surface hydroxyls. On hydrolysis the single SiOH hydroxyls are again regenerated. There is, however, no evidence in the infrared spectrum for the existence of Al-OH groups on the surface of the hydrolyzed solid. Aluminum trimethyl also reacts with all the available surface hydroxyls and in some cases with surface siloxane linkages. In agreement with previous work, the results support a thesis that this reaction occurs in two stages. The first corresponds to reaction of aluminum trimethyl vapor with the surface hydroxyls producing Si-O-Al-Me 2 and ▪ species at the surface, while the second corresponds to a further reaction of these species with neighboring surface Si-O-Si linkages resulting in the presence of both SiMe and AlMe groups on the ultimate surface. The changes in the infrared spectrum of the AlMe 3-treated silicas on exposure to excess water vapor are explained in terms of the relative hydrolytic stability of the Al-Me and Si-CH 3 surface species. Once again there is no clear evidence from the infrared spectrum of the hydrolyzed solid to suggest the presence of surface Al-OH species.