When the coupling agent is dissolved homogeneously in water, the hydrolysis rate of the alkoxy group is influenced by the pH of the solution, the size of the alkoxy group, and to a lesser extent, the size of the organofunctional group directly attached to the silane atom. We have quantitatively studied the reactions of γ-GPS in aqueous solutions. In a dilute solution, the silanol group slowly condensed to siloxane at pH = 4. The epoxy group reacts with both water and the silanol group. The reaction rate of the epoxy group with water is influenced by the pH of the solution. The reaction of the epoxy group with water proceeds faster than with the silanol group due to the large excess of water present, although the rate constants of these two reactions are comparable.