Factors affecting the coverage of two organosilanes, N-2-aminoethyl-3-amino-propyltrimethoxysilane (1) and 3-glycidoxypropyltrimethoxysilane (2), bonded to silica have been investigated. The coverage of 1 is much higher than that of 2 for surfaces modified under identical reaction conditions. This is attributed to the rapid adsorption of 1 via specific interaction between the ethylenediamine group and the silanol groups of the silica surface. The difference in coverage is not simply due to pH differences. The nature of the side-chains of such silanes and the nature of the surface to be modified must be considered in the development of a derivatization scheme.