Methyl chlorosilane vapours react with the surface hydroxyls of Aerosil silicas at temperatures above about 250°C. Infra-red spectroscopic studies on deuterium exchanged samples show that only the external hydroxyl-groups are affected. The trimethyl- and dimethyl-substituted chlorosilanes react selectively but completely with the isolated or single surface hydroxyls, whereas the monomethyl and tetrachloro compounds also react with some of the interacting hydrogen-bonded surface groups. Analyses of the solid reaction products from the dimethyl and trimethyl chlorosilane + silica reactions suggest that these compounds react largely 1 : 1 with the single surface hydroxyls although 1 : 2 bridging reactions occur on preheating the solid to temperatures of the order of 500°C. By comparing these results with those reported earlier1 it is suggested that the selectivity of reactions of this type is controlled largely by the stereochemistry of the activated transition-state structure of the organic molecule.