The adsorption and reaction of ethanol on the metal-free, pure silicon dioxide surface have been studied as a model system to explore the chemical origin of the defects which are detrimental in the gate oxide exposed to chemical vapors. In addition to molecular adsorption, ethanol decomposed to the extent that a variety of species including ethyl, ethoxy, and hydroxyl were produced even at the surface temperature of 115 K. The silicon dioxide surface was selective for the formation of acetaldehyde through cleavage of the C-H bond. The bare surface, however, was more selective for ethanol conversion to ethylene than the surface covered by fragments and products. Comparison of static secondary ion mass spectra taken from the surface exposed to deuterated and nondeuterated ethanol, respectively, showed that the C-H, Et-OH, and EtO-H bonds were disrupted during the surface decomposition of ethanol. Hydroxyl hydrogen of ethanol readily exchanged with silanol hydrogen of the surface. The formation of Si-H and SiO-H bonds suggests that hydrogen concentration will increase on the metal-free silicon dioxide surface exposed to organic vapors. Its implication to the generations of defects in the silicon dioxide framework and of the stress-induced leakage current will be discussed.