The adsorption of 3-glycidoxypropyltrimethoxysilane (GPS) onto silica colloids (20 nm diameter) suspended in an isopropanol/water solution was studied and related to the microstructure and properties of coatings prepared from the suspensions. 29Si NMR results indicate an adsorption limit of ~2.2 GPS monomer or repeat units per nm2 of silica surface after 24 h in suspension. For coatings dried at 100°C, FTIR data show that surface silanols are further consumed as GPS is added beyond the adsorption limit in the suspension. The increased adsorption after drying may be related to enhanced reaction at the higher temperature and to drying, induced deposition onto the surfaces. Coating microstructures consist of randomly packed silica colloids with varying amounts of siloxane polymer filling the pores, depending on the amount of silane added. The critical cracking thickness of the coatings increased dramatically when the amount of silane added was sufficient to fill the porespace. Abrasion resistance of the coatings improved gradually with GPS content, but deteriorated when the GPS content was far in excess of that needed to fill the pores. Hardness measured by nanoindentation showed a similar trend as abrasion resistance.