Reactions of 3-glycidoxypropyltrimethoxysilane (GPS) in a water-rich environment with silica colloids were studied. Si-29 NMR spectroscopy was used to monitor quantitatively the variation in GPS hydrolysis, condensation, and adsorption with pH and water concentration. The results show the competition between adsorption on the silica surface and condensation polymerization of GPS in the bulk liquid and are used to identify conditions favorable to surface modification. The presence of colloidal silica was shown to increase the apparent GPS dimerization rate, suggesting either surface catalysis or an increased local reactant concentration in the electrical double layer around the silica colloids.