The grafting of different silanes and probe molecules on ULTRASIL? 7000 GR was studied with a special IR technique. A relationship between the molecular size of silanes and reacted silanols at the silica surface was determined. The accessibility of the SiOH-groups to the silane is decreasing with the increasing size of its alkyl group (stearic hindrance). VP Si 363? behaves different due to the additional adsorption possibilities of the long alkylpolyethergroups. The combination of ULTRASIL? 7000 GR with VP Si 363? shows the highest efficiency in silica/ silane coupling.