Three mesoporous silica gels with variable pore size were modified with gamma-aminopropyltriethoxysilane. The degree of surface hydration and hydroxylation and the available specific surface area were varied by a variation of pretreatment temperature. The effect of surface and structural properties of the silica substrate on both chemical and physical interaction of the silane with the silica were characterized using spectrophotometric and elemental analysis data. Total deposition is governed by the degree of hydration and the specific surface area of the substrate. Hydration causes hydrolysis and polymerization of the grafted molecules. On the dehydrated but hydroxylated substrate, a coating layer with a coverage of 2.05 molecules/nm(2) is formed. The degree of chemical bonding is controlled by the number of surface hydroxyls if a relative excess of silane over surface silanols is used. On dehydroxylated silica a diverging reaction behavior is observed. The relative amount of physisorption increases largely. If the average pore radius r less than or equal to 20 Angstrom, steric hindrance lowers the surface coverage value.