An infra-red study of the reactions of AlMe3 and TiCl4 with silica surfaces carrying both hydrogen bonded (type B) and single (type-A) surface hydroxyls shows that the former are the more reactive at room temperature. Heating the silica to 700°C dehydrates the type-B sites producing surface siloxane bridges (type-S sites), which are more reactive towards AlMe3 at room temperature and SiMe3Cl at 300°C, than the remaining type-A surface hydroxyls.