The molecular structures of the hydrolyzates of γ-aminopropyl-triethoxysilane (γ-APS) were studied by Fourier transform infrared spectroscopy (FT-IR). The molecular structure of γ-APS is a function of the pH of the solution and the drying conditions. In the partially cured film, cast from a 20% by weight γ-APS aqueous solution at pH = 2, the amine group exists as a protonated amine; and at pH = 12 the amine group exists as a free NH2. The amine group exhibits different structures subject to drying conditions when prepared at natural pH. The sample dried in nitrogen exhibited hydrogen bonding between the SiOH and the NH2group. Upon heating, the silanol groups condense and the amine groups no longer hydrogen bond. The increase in the availability of free amine groups was indicated by larger amounts of exchange of NH2to ND2when the sample was exposed to D2O vapor. When the hydrolyzecl γ-APS at natural pH was dried in CO2or in air, a bicarbonate structure was formed, causing the appearance of bands at 1630, 1575, 1488, and 1332 cm−1. These bands were previously associated with the intramolecular zwitterions of Sio−…NH3+E-glass fibers were treated with 2% γ-APS and dried under different conditions, which show the various structures of γ-APS as described above. Differences in γ-APS structures may affect the curing reactions of composite materials.